Comcast router not providing REAL port forwarding

I have the ARRIS Group, Inc. TG3482G Voice and Data Modem. I recently got it from an 'upgrade' as a rental. prior to that i owned my own Motorola modem. that modem i could actually forward a port, for example, send port trafic from 34565 to 25565. on the new Comcast provided moden, all i can manage to do is pass-though port traffic, even though it says forwarding. are these ARRIS modems just inferior or is there a way to ACTUALLY forward a port on them such as some comination between port forwarding and port triggering?