MongoClient cannot connect to mongo in another Docker container - node.js

Question

I am currently trying to deploy my app with docker but I have some problem with mongodb.
My app is not succeeding to connect to mongodb in another container.
Here is my docker-compose.yml file:
version: "3.7"
services:
mqtt_broker:
build:
context: .
dockerfile: Dockerfile
container_name: mqtt_broker
ports:
- "4040:4040"
links:
- mqtt_db:mqtt_db
depends_on:
- mqtt_db
mqtt_db:
build:
context: ./mongo
dockerfile: Dockerfile
container_name: mqtt_db
ports:
- "27017:27017"
Here is the Dockerfile for the mongo container:
FROM mongo:latest
COPY init_docker.js /docker-entrypoint-initdb.d/
CMD mongod --replSet "rs0" --bind_ip_all
Here is how I connect to mongo in the app container:
const client = new MongoClient(config.get("dbURL"));
const mongo = await client.connect();
const db = client.db(config.get("dbName"));
with the configuration file:
{
"cors": {
"origin": "*"
},
"clientPort": 4040,
"mqttPort": 1883,
"dbURL": "mongodb://mqtt_db:27017",
"dbName": "mqtt_proxy"
}
Here the error I get with the mongo node client:
MongoServerSelectionError: Server selection timed out after 30000 ms
at Timeout._onTimeout (/broker/node_modules/mongodb/lib/sdam/topology.js:305:38)
at listOnTimeout (node:internal/timers:559:17)
at processTimers (node:internal/timers:502:7) {
reason: TopologyDescription {
type: 'Unknown',
servers: Map(1) {
'mqtt_db:27017' => ServerDescription {
_hostAddress: HostAddress { isIPv6: false, host: 'mqtt_db', port: 27017 },
address: 'mqtt_db:27017',
type: 'RSGhost',
hosts: [],
passives: [],
arbiters: [],
tags: {},
minWireVersion: 0,
maxWireVersion: 13,
roundTripTime: 35.24000000000001,
lastUpdateTime: 13896371,
lastWriteDate: 0,
topologyVersion: {
processId: ObjectId { [Symbol(id)]: [Buffer [Uint8Array]] },
counter: 0
},
logicalSessionTimeoutMinutes: 30
}
},
stale: false,
compatible: true,
heartbeatFrequencyMS: 10000,
localThresholdMS: 15,
commonWireVersion: 13,
logicalSessionTimeoutMinutes: undefined
},
code: undefined,
[Symbol(errorLabels)]: Set(0) {}
}
I can actually ping mqtt_db, the ip is resolved, but I don't see it in /etc/hosts.
I also tried to add networks in my docker-compose but it translated mqtt_db in localhost and it does not work.
Here is the repo of my project: https://gitlab.com/mqtt-broker-sniffer/sniffer
EDIT
I had to specify ?directConnection=true when I was connecting to the db.
Crédit: https://stackoverflow.com/a/70204195/19127936
const client = new MongoClient(
`${config.get("dbURL")}/${config.get("dbName")}?directConnection=true`
);
this.client = await client.connect();
this.db = client.db(config.get("dbName"));