I had rebooted my microcell multiple times, spoke with support multiple times, hearing everything from the microcell systems having problems with the iPhone 5, to all microcells in florida having problems, etc. power cycle all equipment (modem, routers, microcell) again. they finally recommended me getting a new SIM card from a corporate AT&T store. I reluctantly did, thinking it wasn't going to work. The other iPhones in my house worked fine with the Microcell (iPhone 4 and 4S).
Then what do you know. Came home after having SIM card replaced, power cycled microcell and iPhone 5, and a few minutes later, finally got the Microcell tag.
Try rebooting the microcell and power cycling your iPhone 5. If an AT&T corporate store isn't far from you, get your SIM card swapped out and try the above again.