Bought used extreme. Factory reset twice. Held reset button as I plugged it in. Set up and connects great at first. It's centrally located in single story home. If I get far away from the router (stay connected to wifi) or leave and come back, it will only reconnect at N. Not AC. This is based on Airport Utility app on phone. I can get AC connection again if I toggle wifi off/on on phone or restart either extreme or phone. Normal? Thanks.