We've had 2 different ARRIS models from Time Warner (Hawaii). You don't actually do regular Port Forwarding. They set up some type of "VIRTUAL" thing that is weird. Asking Time Warner Tech Support is useless, as they insist they never have any complaints, but a quick Google search proved them wrong. We've done Port Forwarding, Open Ports, set up Static IP, UPnP and even DMZ, but would still have problems. We'd be fine then have NAT issues with some people randomly. The best thing I could suggest is to reset the router to factory settings, just Google it (you need a paper clip), have a wired connection (correct settings on console), set up a Static IP, then Enable UPnP. You should be set tjen. This is the cleanest way to see if you have another problem then you can go from there, if you need to.

As long as you are using UPnP, and are getting a NAT Type 2 or 1, you shouldn't need to open ports. If you are connecting with wifi, switch to a wired connection.