If Cellular has stopped working immediately after performing a system software update, the first step is to try a forced restart of your iPad:
Force restart iPad (models with Face ID or with Touch ID in the top button)
If your iPad doesn’t have the Home button, do the following:
- Press and quickly release the volume button nearest to the top button.
- Press and quickly release the volume button farthest from the top button.
- Press and hold the top button.
- When the Apple logo appears, release the top button.

If the issue persists after the forced-restart, the next step is to reset the iPad to factory settings. As this reset will erase all locally stored data and settings you should ensure that you have an iCloud or iTunes backup before performing the reset:
How to back up your iPhone, iPad, and iPod touch - Apple Support
When you are happy to proceed, you can perform the factory reset:
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
If you are remain unsuccessful, you would be best advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Otherwise, you might visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - and you can explore your available options:
Genius Bar Reservation and Apple Support Options - Apple