The AT&T has connectivity problems when it is used with the iPhone 5s and iPhone 6. Users who have updated the iPhone 4 to iOS 6 have also experienced the same problem with connecting to the MicroCell.
The connectivity issues with the iPhone 5 arises because MicroCell does not support LTE connections. The HSPA base band chip on the device's circuit board is incompatible with LTE, so software or firmware fixes are not able to address this problem.
There are several reports that state that to resolve the connectivity problem on the iPhone 5S, the user simply has to turn off the LTE. The works because the MicroCell automatically chooses the fastest connection available, which means that it chooses a weak, unsupported LTE signal, over a stronger 3G or 4G HSPA signal that is supports. Since LTE is not supported by the device, turning it off lets the phones connect to a HSPA signal.
In order to avoid interference with local cell towers, the MicroCell adjusts the generated signal strength based on how weak the actual cell signal is at the time of initial activation. Individual phones adjust their transmitting power depending on the signal strength from the MicroCell. This balancing of signals results in connectivity problems in iOS 6.