Cellular error

To update the cellular settings, have a look at the steps in this article:

Manually update your carrier settings on your iPhone or iPad - Apple Support

Or set it back to factory settings on your computer and restore from the latest backup afterwards, which will also download all needed updates.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Can you give us more details about the issue?

Did you already have a look at these articles?

If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support

If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support