This fixed me up.
'''If you are connected to a wifi network, you are using the wifi. Are you stating that your cellular data only works when it's enabled but not connected to a wifi network? Do you see the 4GLTE icon at the top of your screen? Check Settings>Wirleless&networks and ensure the state of the cellular network is enabled. If it is and the cellular data is not working you can try to reset the network information by opening up your dialer and entering ##72786# and tapping yes. This will take you thru the activation steps once more and you should have your cellular data working once more'''