In order for players to connect to your server you first need to assign a port to it. Fortunately, as part of our system we set a port for your gameserver to use to avoid port clashes etc and our API returns this as part of the standard flow.
For Unity gameservers; set "-port $$port$$" as a command line parameter on the build configuration.
Unreal gameservers work by ensuring that you have a command line parameter of; "-port=$$port$$" in your build configuration.
Both of these should be set by default on any new build configurations that you create, in the event that this gets removed for any reason you can simply recreate them as previously described.