My ATT Microcell works for a few hours and then stops. The configuration is:
Internet -> Comcast Modem -> Time Capsule/Airport Extreme -> MicroCell
Note that the Comcast Modem is providing both my telephone service and a single RJ-45 network connection into the Time Capsule.
The AT&T Tech says that perhaps changing some configuration variables on the TC would do the trick. There are a list of them but lets start with "IPSec Pass-Through is Enabled". I can't find that setting and many others in the Apple Airport Utility.
I know that Apple likes to hide the more geeky options. Is that option changeable on a TC?