Support Cube Universe: Online multiplayer help
How can I play on a server on the Internet?
If you want to play on a server on the Internet, just add its address in the server selection. If that does not work, you may have to ask the person who is hosting the server for help (see below).
How can someone join my server over the Internet?
Tell them to connect to your computer using your public IP address, which currently is 188.8.131.52. In most cases, you have to enable port forwarding (Wikipedia) in your router. You can see the ports used by the server in the server creation in-game.
Your IP address may be changing from time to time, so if you want to host a server regularly, you may want to use Dynamic DNS (Wikipedia).