Go to Settings, turn on the Airplane mode, and turn it off again.
Press and hold the sleep/wake button, and drag the slider when appears to turn off your device. Turn it on again.
Navigate to Settings, click on General > Reset > Reset Network Settings.
Go to Settings, tap on Cellular, choose on Enable LTE, and click on Data Only.
To do this, make sure you are connected to the Internet. Click on Settings, General, and then About.