I have an AT&T Microcell as well but I do not have it connected in 'passthru' mode. I'd heard too many problems doing it that way and I'm surprised you got yours to work.
I connect mine as just another client on the network. I set up port mapping with public and private UDP ports 123,4500,500,4500 and public TCP port 443. I gave the Microcell it's own private IP address as well via a reserved DHCP address based on it's MAC address.
So basically, if the microcell has to reset it will get the same IP address every time (allowing it to operate in DHCP mode). And this IP address is Port mapped by enabling the NAT port mapping protocol so that everything just works. I've NEVER had a problem with the setup. and it allowed me to move the microcell to a south facing window (to get a good GPS signal) and place it anywhere in the house, not just near the cable modem.
Hope this helps.
If you need more info, just ask. The Microcell allows me to have full coverage on all my cell phones anywwhere in my house.