Clustering nodes that are on different servers

I'm using this docker image and trying to cluster 2 nodes that are on different servers, therefore 2 different public IPs.
Just for testing, I successfully clustered 2 docker containers that are on the same machine.

However, when I try to define a FQDN in ERLANG_NODE_ARG, I get an error that I don't know how to overcome.

This container starts without errors (I'm skipping unrelated lines):

services:
  ej1container:
    hostname: ej1container          # containername works here too
    environment:
      - ERLANG_NODE_ARG=ej1@ej1container

This setup gives me an error

services:
  ej1container:
    hostname: ej1container          # containername works here too
    environment:
      - ERLANG_NODE_ARG=ej1@subdomain.domain.com

It looks like the container starts normally but when I do

docker exec ej1container ejabberdctl status

I get

Failed RPC connection to the node 'ej1@subdomain.domain.com': nodedown

I already pointed the A record of subdomain.domain.com to the public IP of the VPS where this is running.

There was a similar issue https://github.com/processone/docker-ejabberd/issues/106 but I don't see how the FQDN was integrated and what the solution was.

Any help would be much appreciated.

Start by inspecting how the Docker image handles ERLANG_NODE_ARG and how the ejabberdctl status entry point connects to the node. Reproduce the Docker Compose configurations with ej1@ej1container and ej1@subdomain.domain.com, then trace the cross-server Erlang networking setup. Done means the FQDN-based nodes cluster successfully and ejabberdctl status returns normally.