It looks powerful, but I was unable to make a succesful online multiplayer test with my GML2 on one Windows 10. 

Lan works great in sandbox.

When I launch the exe, it keeps launching multiple times which makes it unusable like that. I didn't find the code of launching it multiple times, like at "__init_global", but I have my own other method to launch a game multiple times for multiplayer.

GMnet will try to use UPnP and UDP punch (if you setup the java master server) to make an online connection. But in many cases the host need to do a manual port-forward or use Hamachi with the direct IP connect.

The game open multiple times because of the dual instance object (for testing in the IDE). So you need to remove the dual instance object from the first room before you build.