You can also use dynamic dns and forward its port via a virtual server on the router (if the router capable to do it) and the other players had to change their default ('player') name by editing one of the class files with a hex editor inside one of the java archive files. Then it would work in an unofficial way. I played it like this about a year ago with friends, so I don't know what about the latest versions of this game.
Oh, and I think it won't use any download until the router got an incoming packet from the outer dyndns server to let the users through. I still don't know if it works with mobile connections. Cause I assume you'd like to do this via a 3g or 4g modem.