An update is required to use cellular data this iphone
cellu data error and this unavailable to check you update
iPhone 6 Plus, iOS 12
Question marked as
Most likely cause of that message is that the
iPhone is locked to a wireless provider other
than the one whose sim you are intending to use.
What does it say when you look at Settings>General>About>Carrier?
You may have to scroll to see the entry for Carrier.
To check if iPhone is locked to a Carrier, put the sim
you intend to use into the iPhone. If you get no error
message, the iPhone is not locked to a wireless provider.
There is a section in Settings called Carrier Lock.
Settings>General>About and scroll down to Carrier Lock.
If the iPhone is unlocked, it should say 'No SIM restrictions'.
If you get a different answer, the iPhone is locked.