Most Comcast devices these days support bridge mode as well, you can enable it (or have Comcast help you enable it), and it will just act as a modem and push all traffic to your USG.
If it's a combo modem and router device you're likely going to run into issues due to it NAT'ing traffic.
The best route to go would be to buy your own (separate) modem and a network switch. If you need wireless buy an wireless router that supports AP mode or just by a stand alone wireless AP (cheaper). Asus wireless routers work well and I know for a fact they support AP mode.
My current perimeter setup is
comcast internet ---> modem(owned) ---> UTM security appliance
UTM ------------------> wireless router in AP mode
UTM ------------------> network switch for other segments
If the 4 switch ports are enough on a wireless router for any wired devices you have then the extra isn't really necessary
Most Comcast devices these days support bridge mode as well, you can enable it (or have Comcast help you enable it), and it will just act as a modem and push all traffic to your USG.