I have a problem saying cellular update failed

If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart - Apple Support...

Update your carrier settings on your iPhone or iPad - Apple Support...you may need this...

iPhone 7 Repair Program for "No Service" Issues - Apple Support...this may apply to your phone...

If the above does not help the best thing to do right now is contact support, you can click "Contact Support" top right corner of this page and follow the leads until you reach the Phone or Chat options.

They are very busy right now so you may have to try a few times, also some options are not always immediately available, but will become available as demand decreases.

Or maybe try here...Contact Apple for support and service - Apple Support