|Btw the cable modem is bridged.|
Bridged to what? Why did you do this?
A port forward is pretty straightforward and simple to do on most any SOHO Router I've worked with. I have no experience with the model you have but it should still be a simple case of forwarding a specific port to a specific LAN IP.
game port = 4500
IP of PC hosting game (in server mode) = 192.168.1.10
Forward in router should look as follows:
Forward port 4500 to 192.168.1.10
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.