I like to ask questions since it's very difficult to diagnose over the internet.
are you rooted?
has this happened from day one of using the device?
did you install or uninstall anything prior to this issue?
does the phone lock up globally? meaning when on cellular data can you use the phone to make calls, text, use the browser, play locally stored video or audio files?
does it happen everywhere or in certain areas?
seems its only occurring when in 4G areas? is this correct?
if you are in a fringe 4G area and the phone is constantly switching back and forth between 3G and 4G, this is most likely the reason.