When your server is booted, you want to register it on a third-party service; the external service can't reach your server. It is usually a port register problem (Just like figure A).
Arbitrium can't reach the application ports directly from any external source. The ports are mapped automatically to the external, and you should register the server with the NATed ports. It's possible to retrieve the ports from your deployment. You will need to use ARBITRIUM_CONTEXT_URL and ARBITRIUM_CONTEXT_TOKEN to get the information form your container itself. Here's a python script that retrieves the data.
It would be best not to change the ports your game server is listening to. Just like figure B: