Simply toggle Airplane Mode to On for ten seconds, then back Off. This should cause your device to properly switch from Wi-Fi to LTE/4G.
This is a known issue with iOS but does not effect all users in all cases. Apple is aware of the issue, and we hope it will be fixed in an upcoming software update. If you still have *any* data carrier connection problems after toggling to Airplane mode, please shoot us a note at firstname.lastname@example.org so we can investigate.