Running an application within AMP is no different from running it outside of AMP in terms of connectivity.
If you can access the game from another device on the same network using your internal IP, then the game server, AMP, and the servers firewall are all functioning correctly and configured correctly, otherwise you would not be able to connect. This means that failure to connect externally is down to bad port forwarding.
If you cannot access it from another device on the same network, then something is wrong with the local firewall configuration.
Have you checked that ampinstmgr dumpfirewall shows all the expected firewall rules? And that the Windows Advanced Firewall control panel reflects that?