Hi Brian,
I was told about the bridge mode as well by the installation tech or a phone/chat support person, but since I was confused as to why I needed to put in bridge mode (since it would disable the router), I did some digging and found that the solution I posted was the correct one.
I have my own modem & router behind the Comcast-supplied security router now, with my router in the DMZ of the security router, which still gives me full access to my router (I can still run VPN & Ooma with my personal router). I still don't have acess to the Comcast security router, but at least with my router on its DMZ, I have pretty much full access as I did before.
The other way you can do it is put the Comcast security router on your router's DMZ-that in theory should work as well.