Port forwarding.. is not a guaranteed part of the service... that being said.. its something simple, and generally SHOULD work.
(i am really suprised they havent updated the firmware to fix it... but i guess they are pushing the CGN3)
You do have potentially a few options, depending on which way you want to go about it.
A ) Do you have or used to use, a 3rd party router with just a modem? You can set up the CGN2 in BRIDGED MODE, efectively turning it into a modem only, then use the port forwarding, etc on the router itself instead.
B ) Which internet package are you on? In most cases, you should be able to swap modems around.. You can call in and ask if they can give you one of the other ones, the SMC or Cisco, in which the port forwarding does work.