Cellular data
What do I do if the cellular data on my phone says an update is required to use cellular data on this iPhone ?
iPhone 11, iOS 14
If you already tried (to no avail) all the steps provided in this support article:--> If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart - Apple Support, I recommend that you make an appointment and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector
After checking it out, they'll give you all the available options.
Regards
Giulio