Timed Out when connecting to device via L2CAP bluetooth socket, when LTE chip is integrated - sockets

Question

I'm experiencing a strange issue with one of my applications:
Background:
One of my features requires Bluetooth connectivity. Once a device (i.e. phone) is paired with the host, I continuously "poll" the device, to extract the RSSI (signal strength).
The polling is achieved by connecting to the device on PSM port 0x0001 (1), via Bluetooth sockets on L2CAP, sending an HCI request to query for signal strength, extracting the signal strength, and closing the connection.
Issue:
Everything works fine, and as expected, until I integrate the Simcom 7600G LTE module (Needed for internet connectivity) via PCI.
When I plug the module in, I am no longer able to do an L2CAP Bluetooth socket connection, with an error message of "Time Out" (it will time out depending on what I set the socket.settimeout(...)
As soon as I unplug the LTE module, everything works as expected and I am able to establish the socket connection with the remote Bluetooth device.
Observation:
What is strange is that the Bluetooth discovery and pairing process still works as expected, with the LTE module integrated. It is only the L2CAP Bluetooth sockets that I've written in code (Python) that no longer work, timing out every time a connection is attempted.
So, simply, when the LTE module is not in the PCI slot, everything works flawlessly - the discovery, pairing, BluetoothSocket L2CAP connection, and RSSI querying.
When it is plugged out, the discovery and pairing work, but I get Timed Out when attempting to establish a BluetoothSocket connection via L2CAP. (I've also tried RFCOMM with no luck).
Another strange observation is, when I am monitoring bluetoothctl, I can see the Agent displaying:
[CHG] Device BB:BB:BB:BB:BB:BB Connected: yes
[CHG] Device BB:BB:BB:BB:BB:BB Connected: no
And it displays this in both scenarios, with the LTE module in and out, yet at the application layer, it times out.
Questions and Assumptions:
As a general question, why might this be occurring?
Could it be some form of interference? And if so, given other protocols seem to be working fine, with all other protocols presumably on the same band (2.4GHz), why is it that L2CAP is affected? As far as I understand, the Bluetooth Stack specifies L2CAP as the underlying layer 3 protocol.
If not interference, what other settings could be interfering between
two completely different chips, and network interfaces?
Additional Data:
Test code:
import traceback
import bluetooth
import bluetooth._bluetooth as bt
bt_sock = bluetooth.BluetoothSocket(bluetooth.L2CAP)
bt_sock.settimeout(5)
# [0, 672, 65535, 0, 1, 3, 63]
print(bt_sock.get_l2cap_options())
try:
print("Connecting...")
bt_sock.connect(("BB:BB:BB:BB:BB:BB", 1))
print("Closing...")
bt_sock.close()
except Exception as e:
print(e)
print(traceback.format_exc())
I've run a btmon on my adapter, with the below problem dump:
Bluetooth monitor ver 5.55
= Note: Linux version 5.4.154-5.5.0-devel+git.c65f1622951c (aarch64) 0.210966
= Note: Bluetooth subsystem version 2.22 0.210971
= New Index: AA:AA:AA:AA:AA:AA (Primary,USB,hci0) [hci0] 0.210973
= Open Index: AA:AA:AA:AA:AA:AA [hci0] 0.210973
= Index Info: AA:AA:AA:AA:AA:AA (Marvell Technology Group Ltd.) [hci0] 0.210975
# MGMT Open: bluetoothd (privileged) version 1.14 {0x0001} 0.210976
# RAW Open: python3 (privileged) version 2.22 {0x0002} 13.767587
# RAW Close: python3 {0x0002} 13.767635
# RAW Open: python3 (privileged) version 2.22 {0x0002} [hci0] 13.767704
< HCI Command: Create Connection (0x01|0x0005) plen 13 #1 [hci0] 13.771761
Address: BB:BB:BB:BB:BB:BB (OUI AC-6C-90)
Packet type: 0xcc18
DM1 may be used
DH1 may be used
DM3 may be used
DH3 may be used
DM5 may be used
DH5 may be used
Page scan repetition mode: R2 (0x02)
Page scan mode: Mandatory (0x00)
Clock offset: 0x0000
Role switch: Allow slave (0x01)
> HCI Event: Command Status (0x0f) plen 4 #2 [hci0] 13.776071
Create Connection (0x01|0x0005) ncmd 1
Status: Success (0x00)
> HCI Event: Role Change (0x12) plen 8 #3 [hci0] 14.798704
Status: Success (0x00)
Address: BB:BB:BB:BB:BB:BB (OUI AC-6C-90)
Role: Slave (0x01)
> HCI Event: Page Scan Repetition Mode Change (0x20) plen 7 #4 [hci0] 14.804429
Address: BB:BB:BB:BB:BB:BB (OUI AC-6C-90)
Page scan repetition mode: R0 (0x00)
> HCI Event: Connect Complete (0x03) plen 11 #5 [hci0] 14.804676
Status: Success (0x00)
Handle: 1
Address: BB:BB:BB:BB:BB:BB (OUI AC-6C-90)
Link type: ACL (0x01)
Encryption: Disabled (0x00)
< HCI Command: Read Remote Supported Features (0x01|0x001b) plen 2 #6 [hci0] 14.804842
Handle: 1
> HCI Event: Command Status (0x0f) plen 4 #7 [hci0] 14.806679
Read Remote Supported Features (0x01|0x001b) ncmd 1
Status: Success (0x00)
< HCI Command: Write Scan Enable (0x03|0x001a) plen 1 #8 [hci0] 14.806804
Scan enable: No Scans (0x00)
> ACL Data RX: Handle 1 flags 0x02 dlen 10 #9 [hci0] 14.816323
L2CAP: Information Request (0x0a) ident 2 len 2
Type: Extended features supported (0x0002)
> HCI Event: Max Slots Change (0x1b) plen 3 #10 [hci0] 14.822810
Handle: 1
Max slots: 5
> HCI Event: Read Remote Supported Features (0x0b) plen 11 #11 [hci0] 14.826428
Status: Success (0x00)
Handle: 1
Features: 0xbf 0xfe 0x8f 0xfe 0xdb 0xff 0x7b 0x87
3 slot packets
5 slot packets
Encryption
Slot offset
Timing accuracy
Role switch
Sniff mode
Power control requests
Channel quality driven data rate (CQDDR)
SCO link
HV2 packets
HV3 packets
u-law log synchronous data
A-law log synchronous data
CVSD synchronous data
Paging parameter negotiation
Power control
Transparent synchronous data
Broadcast Encryption
Enhanced Data Rate ACL 2 Mbps mode
Enhanced Data Rate ACL 3 Mbps mode
Enhanced inquiry scan
Interlaced inquiry scan
Interlaced page scan
RSSI with inquiry results
Extended SCO link (EV3 packets)
EV4 packets
EV5 packets
AFH capable slave
AFH classification slave
LE Supported (Controller)
3-slot Enhanced Data Rate ACL packets
5-slot Enhanced Data Rate ACL packets
Sniff subrating
Pause encryption
AFH capable master
AFH classification master
Enhanced Data Rate eSCO 2 Mbps mode
Enhanced Data Rate eSCO 3 Mbps mode
3-slot Enhanced Data Rate eSCO packets
Extended Inquiry Response
Simultaneous LE and BR/EDR (Controller)
Secure Simple Pairing
Encapsulated PDU
Erroneous Data Reporting
Non-flushable Packet Boundary Flag
Link Supervision Timeout Changed Event
Inquiry TX Power Level
Enhanced Power Control
Extended features
> HCI Event: Command Complete (0x0e) plen 4 #12 [hci0] 14.834553
Write Scan Enable (0x03|0x001a) ncmd 1
Status: Success (0x00)
< HCI Command: Read Remote Extended Features (0x01|0x001c) plen 3 #13 [hci0] 14.834600
Handle: 1
Page: 1
> HCI Event: Command Status (0x0f) plen 4 #14 [hci0] 14.836925
Read Remote Extended Features (0x01|0x001c) ncmd 1
Status: Success (0x00)
> HCI Event: Read Remote Extended Features (0x23) plen 13 #15 [hci0] 14.845426
Status: Success (0x00)
Handle: 1
Page: 1/2
Features: 0x0f 0x00 0x00 0x00 0x00 0x00 0x00 0x00
Secure Simple Pairing (Host Support)
LE Supported (Host)
Simultaneous LE and BR/EDR (Host)
Secure Connections (Host Support)
< HCI Command: Remote Name Request (0x01|0x0019) plen 10 #16 [hci0] 14.845484
Address: BB:BB:BB:BB:BB:BB (OUI AC-6C-90)
Page scan repetition mode: R2 (0x02)
Page scan mode: Mandatory (0x00)
Clock offset: 0x0000
< ACL Data TX: Handle 1 flags 0x00 dlen 10 #17 [hci0] 14.845502
L2CAP: Information Request (0x0a) ident 1 len 2
Type: Extended features supported (0x0002)
< ACL Data TX: Handle 1 flags 0x00 dlen 16 #18 [hci0] 14.845526
L2CAP: Information Response (0x0b) ident 2 len 8
Type: Extended features supported (0x0002)
Result: Success (0x0000)
Features: 0x000002b8
Enhanced Retransmission Mode
Streaming Mode
FCS Option
Fixed Channels
Unicast Connectionless Data Reception
> HCI Event: Link Supervision Timeout Changed (0x38) plen 4 #19 [hci0] 14.845680
Handle: 1
Timeout: 5000.000 msec (0x1f40)
> HCI Event: Command Status (0x0f) plen 4 #20 [hci0] 14.847801
Remote Name Request (0x01|0x0019) ncmd 1
Status: Success (0x00)
> HCI Event: Remote Name Req Complete (0x07) plen 255 #21 [hci0] 14.859923
Status: Success (0x00)
Address: BB:BB:BB:BB:BB:BB (OUI AC-6C-90)
Name: Sasa's S21 Ultra
# MGMT Event: Device Connected (0x000b) plen 31 {0x0001} [hci0] 14.859948
BR/EDR Address: BB:BB:BB:BB:BB:BB (OUI AC-6C-90)
Flags: 0x00000000
Data length: 18
Name (complete): Sasa's S21 Ultra
# RAW Close: python3 {0x0002} [hci0] 15.848984
< HCI Command: Disconnect (0x01|0x0006) plen 3 #22 [hci0] 17.852261
Handle: 1
Reason: Remote User Terminated Connection (0x13)
> HCI Event: Command Status (0x0f) plen 4 #23 [hci0] 17.853904
Disconnect (0x01|0x0006) ncmd 1
Status: Success (0x00)
> HCI Event: Disconnect Complete (0x05) plen 4 #24 [hci0] 17.867262
Status: Success (0x00)
Handle: 1
Reason: Connection Terminated By Local Host (0x16)
# MGMT Event: Device Disconnected (0x000c) plen 8 {0x0001} [hci0] 17.867286
BR/EDR Address: BB:BB:BB:BB:BB:BB (OUI AC-6C-90)
Reason: Connection terminated by local host (0x02)
< HCI Command: Write Scan Enable (0x03|0x001a) plen 1 #25 [hci0] 17.920401
Scan enable: Page Scan (0x02)
> HCI Event: Command Complete (0x0e) plen 4 #26 [hci0] 17.924903
Write Scan Enable (0x03|0x001a) ncmd 1
Status: Success (0x00)