Well have you tried anything that was suggested.!  when your phone kicked off of the 4G and went to 3G when that happens the phone needs to re-establish with the Network So pulling out Sim while the phone is off for about 10 min. then re-installing it should re-establish the Connection if not the Sim Cards are FREE.. Just ask for another One) And one other thing you can try since the phone is considered a Global phone it has a switch that is Located with the LTE / CDMA go into Settings > Data > More Then Go to Mobile Networks > then go to Network Mode and Check the LTE / CDMA feature.. Tap the home button see if that works..