If this modem has a built in router, it's best to bridge the modem. Having 2 routers on the same line can cause connection problems.
Double NATTo tell if the modem is bridged or not, look at the routers web page, Status/Device Info/Wan Section, if there is a 192.168.0.# address in the WAN IP address field, then the modem is not bridged.
If the modem can't be bridged then see if the modem has a DMZ option and input the IP address the router gets from the modem and put that into the modems DMZ.
I would recommend that you use Advanced/Virtual Server and input the IP address of the PC that your using for gaming and use the WoW ports that is needed for the game.