AT&T iPhone 5 gets stuck on EDGE network

When my AT&T iPhone 5 goes to a low signal zone, or an EDGE zone, such as a friends basement, my girlfriends house, or an elevator, it switches to EDGE, but when I go to a good signal area, it stays on EDGE for a while (at least an hour), or until I shut off the antennas using airplane mode or restarting the phone. Sometimes it also affects Wi-Fi, and when the phone connects to Wi-Fi it gets no data on some apps, until I reset the connection using airplane mode. I want to know once and for all if this is a known hardware or software issue, if I should try to get the phone replaced (which I've had since October), or I should just deal with it as a new phone will not help. I've tried reinstalling iOS, resetting network connections, and DFU restore.