Port forwarding has never worked since I was upgraded to the G3100. I purchased it to go with the FIOS ONE and it was a stupid move. I did everything. Under Firewall, Port Forwarding and Port Triggering, I submitted a rule to allow port 9090 for CML communications. I was not able to access my interior network. I even set a rule under Network Connections, Port Forwarding. Nothing. For over a year, I've had this problem and Verizon and Altius have never been able to fix or suggest a work around. Now I'm being offered the CR1000a. If I get this router and it doesn't work, I'm going back to Comcast or buying my own router. Since the G3100 and CR1000a block all ports inbound by default, I believe there may be issues with certain device when attempting port forwarding. They actually need to add an entire remote access suite with splash page such as VPN or RDP services. Now I have to sell this G3100 to an unsuspecting dupe.
Which port are you trying to forward? Port 9090? Is it TCP or UDP transport? That should work on G3100. Could you screenshot your Port Forwarding page under Firewall?