iPhone stuck on edge

I having a weird problem with the iPhone 5. When I am in a building where the reception is poor, the device picks up edge network signals. But when I leave the building and move to an area where I know there are strong 4g signals, the device still clings to the edge network. The only solution is to go to airplane mode and back or reboot. Does this sound like a software issue or a hardware issue? ATT has replaced the sim and reprovisioned the account and Apple has replaced the device once. It's still not fixed.