The port can be changed by adding "+set net_port 30721" - or whatever port you wish to use - to the command line.
Regarding your port forwarding, it sounds like that was not done correctly. There are entirely too many variables to go through to figure out what it might be, but the basics:
*) The port must be UDP, not TCP, in the forwarding rules
*) Outgoing traffic should get through fine, so you should only need to modify incoming rules
*) The idea is to get traffic from the Internet, that hits your router on that port, to forward to the internal IP address of your server on whatever port is listening.
In many cases, a Google search for your model router plus the words "port forwarding" will return results matching how to do this with your router, which you may need to adapt for the specific task at hand.