so me and my friend have been trying to host novetus servers for like months now and its not working, it shows this weird error thing saying Failed to connect to the Game. (ID=15)

Thats because no one on either the novetus team or online has ever given a clear explenation for how to do it for someone who isn't familiar with networking and all of that stuff. 

Have you tried turning off UPnP? Some ISPs block UPnP.