Connect docker mongo replica set with localstack container - mongodb

Question

I try to connect two docker containers, one is localstack with lambda function I am invoking, the other one is mongo replica set.I tried connecting them with various network configurations, but I fail to reach the mongo from lambda.
Lambda VPC config is empty: "VpcConfig": {},, I read that it should be if I want to access external network.
Key points:
I can connect to mongo from my rest API app on my host.
I can ping the mongo container from within localstack container.
root#e42146a357e1:/opt/code/localstack# ping db
PING db (172.19.0.2) 56(84) bytes of data.
64 bytes from db.localstack_backend (172.19.0.2): icmp_seq=1 ttl=64 time=0.040 ms
64 bytes from db.localstack_backend (172.19.0.2): icmp_seq=2 ttl=64 time=0.038 ms
...
The error I get from Lambda:
MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017
at Timeout._onTimeout (/tmp/function.zipfile.d9dce9f0/node_modules/mongodb/lib/sdam/topology.js:277:38)
at listOnTimeout (node:internal/timers:564:17)
at process.processTimers (node:internal/timers:507:7) {
reason: TopologyDescription {
type: 'ReplicaSetNoPrimary',
servers: Map(3) {
'127.0.0.1:27017' => [ServerDescription],
'127.0.0.1:27018' => [ServerDescription],
'127.0.0.1:27019' => [ServerDescription]
},
stale: false,
compatible: true,
heartbeatFrequencyMS: 10000,
localThresholdMS: 15,
setName: 'rs0',
maxElectionId: new ObjectId("7fffffff0000000000000001"),
maxSetVersion: 1,
commonWireVersion: 0,
logicalSessionTimeoutMinutes: null
},
code: undefined,
[Symbol(errorLabels)]: Set(0) {}
}
What I don't know here is if the error comes from the server or from the client. I also don't know why in the error servers are set to 127.0.0.1:port.
Connection string which lambda uses: mongodb://db:27017,db:27018,db:27019/?replicaSet=rs0
I tried also with IPs from docker network.
Client connection:
const client = new MongoClient('mongodb://db:27017,db:27018,db:27019/?replicaSet=rs0', {
maxPoolSize: 20,
minPoolSize: 0,
retryReads: true,
retryWrites: true,
});
Docker compose file:
services:
db:
image: candis/mongo-replica-set
ports:
- "27017:27017"
- "27018:27018"
- "27019:27019"
networks:
- backend
container_name: db
localstack:
image: localstack/localstack
ports:
- "4510-4559:4510-4559"
- "4566:4566"
networks:
- backend
container_name: localstack
networks:
backend:
driver: bridge
I was thinking that maybe lambda can't connect with outside world due to VPC configuration.