"An update is required to use cellular data on this iPhone" message.
My phone
[Re-Titled by Moderator]
iPhone 7, iOS 15
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See -> If you see No Service or Searching on your iPhone or iPad
If you see No Service or Searching on your iPhone or iPad - Apple Support
If you still don't have a 'service' after that, see -> iPhone 7 Repair Program for "No Service" Issues
iPhone 7 Repair Program for "No Service" Issues - Apple Support
Then check ⤵️
Manually update your carrier settings on your iPhone or iPad
Manually update your carrier settings on your iPhone or iPad - Apple Support
Update your network provider settings manually on your iPhone or iPad
Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)
Charge the battery > 70%.