hard to say what causes your problem.
I run my GX at my workplace behind a router/modem I got from my ISP.
To run my own security gateway behind this router, it was required to activate IP-Passtrough on it. Means all incomming requests will be forwarded to port 1 on my ISP's router set with a specific IP-address. The GX had to be set to a static IP-address on the WAN-port. Now it handles all request comming from the internet as if there is no other device in between. The router functionality on my ISP's router is still working. I can connect devices directly to it and they get internet access.
At home I have a different ISP. This time the router had to be set to Bridge-Mode, this router acts just like a modem. The personal security gateway is set to optain a dynamic IP from my ISP. Here I can not plug in devices to my ISP's router to get internet access. They all have to be connected to my personal security gateway.
On both locations I have port forwarding rules set. They all work fine.
If you have a spare router somewhere laying around, you could hook that one up, instead of the GX50 and see if port forwarding will work. If it won't work, you might have a problem with the forwarding rules on your comcast router/modem.