The Unreal Engine server process is listening on port 7777. Try to open this port on your server but don’t expect the session discovery to work, it will work only over the same LAN. Just try to connect by IP address to the server. If you use the Steam online subsystem you can leverage a set of services like matchmaking.
Make sure to add these lines to your gameengine.ini to enable the default online subsystem:
[OnlineSubsystem]
DefaultPlatformService=Null