How to play multiplayer

[K_Freddie]

This is your problem... you have a dynamic IP address, which means everytime you switch off your USB/wifi thingy.. the network reassigns the IP addresses.

This means you have to update everyone with your IP address if you're running a server. Joining a server should be no problem unless the network changes IP's at regular intervals.
If running a server, this is where Hyperlobby works better as it always has (should have) your current IP.