I just recently switched an unlocked AT&T HTC One M9 over to a prepaid T-Mobile sim card. After going through the dance of building the correct APN the phone is functioning as it was on AT&T's network except - the stock mobile hot spot feature does not/cannot connect or activate. Are you able to utilize the stock mobile hot spot feature on your phone or was it necessary to root & utilize a 3rd party tethering app? I've been unsuccessfully searching to see if there might be a specific APN code or line within an APN code to allow the hot spot to function as intended. Any help/info is appreciated.