iPad Pro stopped syncing photos with iCloud

If you have adequate iCloud storage - and have already tried disabling/re-enabling iCloud Photos - the next unavoidable step is to fully sign-out from iCloud:

Settings > [Your Name / AppleID] > Sign Out

When signing out, when prompted, accept the opportunity to keep a local copy of your data.

After signing-out, perform a 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:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

After restarting your iPad, sign-in to iCloud from iPad settings.

Have you sufficient iCloud storage? If you have exceeded your iCloud storage quota, regardless of available local storage space on your iPad, you may not be able to synchronise photos and files with iCloud.

All iCloud accounts have just 5GB of storage provided "free of charge". Beyond this, you must subscribe to iCloud+ - from which you can choose a higher "paid" iCloud storage tier:

Upgrade to iCloud+ - Apple Support

iCloud+ plans and pricing - Apple Support