i have the save problem in UAE, the problem comes from the LTE network.
the problem solved as the following:
1- make factory reset and complete format
2- after formating and while activating your phone , you will get message from your carrier saying that there is a carrier network update, this update is for the LTE .
Don't update the carrier and keep using normal or 3G, then the problem will solved.
note:
before update you will see you mobile like this:
setting -general - cellular - cellular data & Enable 3G.
after :
setting -general - cellular - cellular data , LTE, .......etc.