what band uses ATT for LTE 4G? i have same problem but is not the phone, or coverage is the signal, the higher bands 2.600 MHZ-2.100 MHZ and others higher bands have serious problems to reach coverage in indoor environments , in fact i have tested in an good coverage area the 4G in my case in Chile with Movistar and a Nokia Lumia 820 y outside between 2 buildings have 3 bars of signal and the speed is 22,17Mbps but entering to the building the 4G signal fades and just get 3G signal.
the reason of the odd phenomenon can be explained because to an higher band uses a QAM (Quadrature Amplitude Modulation) higher ie QAM 256 which also allow higher payloads or speeds but the sacrifice is in coverage in indoor situations with strong electric impulsive noise, and multipath which the higher bands can not manage, in Chile we uses LTE 2.600 MHZ asiatic band but also are trying of make avalaible the 700MHZ band for indoor reception, an specie of backup band if you enter to your home to enjoy LTE on your phone.
i resume is not the coverage of your provider many times the same band used is the responsible of indoor poor or non existent coverage