Similar issue. Solution recommend above appears to work.
Situation: replacing Comcast Modem/router TG862 with my own modem TM822. Primary issue is the TG862 had 2 ethernet ports so I could have the Netgear security router in one port and my Airport Router in the other. But the new modem, TM822, is a single ethernet port, and not a router. If you do what Comcast wants...put the Security Netgear into the Modem, and then your Airport Router into the Netgear...all sorts of issues ensue and because you cannot see into the Netgear to configure it, solving the issues is hard! And Comcast support are out of their depth.
So following the advice on here, I put the Airport into the modem, connected the security Netgear router into the Airport. Put my airport into DHCP+NAT (remember it was in bridge mode in the original config because the Comcast Modem TG862 was also a router....but the TM822 is not!). And the last clue, provided above, was to reboot the security touch screen. Then under advanced settings on the touch screen you can test connectivity on the network and the cellular...it all works!
Don't get me started on how many calls to Comcast I made over the past 2 days...and after explaining the technical struggle trying to make the config work as Comcast wants, to one support agent, she asked me if I had plugged in the Ethernet cable... &*&^%$#@ You can't make this stuff up!!!
-K