Error code 4000 when update attempted

I've tried to update my iPhone 7S to iOS13.7, but this caused 'Cellular Update Failed' and I now have no mobile capability (but I do still have internet access). iPhone now says it is iOS 13.7. I tried Ctrl-'Check for Update' and got error message "iPhone could not be updated. An unknown error occurred (4000)"

Tried the troubleshooting at https://support.apple.com/en-us/HT201210 but same result. And 4000 is not included in the list of error codes.

What can I do to fix the problem?