Iphone 5 (ios6) can use LTE, but not regular 4g. Anyone have a fix or clue why?

So my iphone 5 the past few days has been not working, says can not connect to cellular network, but my screen clearly has 5 bars and att and 4g on could I could only send and receive SMS, no iMessage no safari and no Internet at all. When I enable LTE everything works fine. Anyone know why this is happening?