CocoaMQTT iOS client cannot connect to a broker with TLS - swift

Question

CocoaMQTT client is running on iOS15 with the SSL enabled as shown in this example. The cert_key.p12 file was merged from the client.crt and client.key files signed by the same (self-created) CA that was used for the MQTT server/broker certificate generation. The MQTT broker is configured to require client's certificate and use its CN as the username. The handshake does not go well - the log complains about the unknown certificate:
New connection from 192.168.1.87 on port 8883.
OpenSSL Error[0]: error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown
Based on the above error research, the broker does not like the client's certificate, right? In fact, looking at the wireshark's log, it seems like the client does not send the certificate to the server. Is that something I need to enable in the App configuration? Does the Swift CocoaMQTT package even support this feature (provide the cert to the broker)?
EDIT:
I tried running a client with the same cert/key with paho mqtt implemented in python, which also allows to provide the client with the CA certificate, and everything runs ok:
Trying using the same client's cert/key in CocoaMQTT implemented in iOS15 shows the MQTT broker log error message as above and the fatal alert shows up the the wireshark log:
Initially, I thought that the sever did not like (or did not receive) the client's certificate, but that fatal alert package destination is port 8883. So now I tend to believe that it's the client, who does not like the server's certificate. This is expected if the client uses the pool of official CAs to verify the certificate instead of recognizing that its own certificate was issued by the same CA as the server's one. This is further confirmed by setting the allowUntrustCACertificate = true and seems like the handshake is suspended and no more communication occurs:
There's also a debug message on the client's side:
Call the SSL/TLS manually validating function
So it looks like the client will not continue the communication until this validation process occurs. As #Brits mentioned in his comment, there's a callback to validate the cert manually but it is implemented as a part of the delegate. I do see that there's a method mqtt.didReceiveTrust which I assume should be used for cert validation, and I wish to use the closures approach as stated on the README page as giving an example for the didReceiveMessage methond:
Now you can use closures instead of CocoaMQTTDelegate:
mqtt.didReceiveMessage = { mqtt, message, id in
print("Message received in topic \(message.topic) with payload \(message.string!)")
}
With a weak understanding of Swift Closures, I am not able to figure out how to make that function all, so the question now becomes: how to convert that function from the delegate definition into the closure?