Port forwarding should be fairly straightforward, but what Arris doesn't tell you is that you need to have the firewall on in order for port forwarding settings to have an effect.
I don't know why the "firewall off" option would mean "block everything", but as best I can tell, that was the case on my Arris DG2470A (from RCN, could be some custom firmware that RCN uses?)
So step by step: