I wouldn't try to use AppGameKit as server software outside of a LAN situation. Thing is, you can't run AppGameKit headless, and servers for rent are all headless. For multiplayer-game over internet, your best bet is to read up on websockets and write the server app in a more suitable language like Go, Python, C# or even Java (if you like hurting yourself).
Now you could run your own server at home (or piggy-back off some office IT infrastructure you got admin access to) and use AppGameKit for the server app, do all the port-forwarding and hook up a UPS to the server and modem - and have a DNS point to your outbound IP. A Raspberry Pi 3 will do fine for limited traffic. But on that, you don't need any special server version of the OS. Ordinary Raspbian Pixel will do fine. Just as on an AMD64/x86 Ubuntu box, regular plain vanilla Ubuntu will do just fine.