Restart your iPad, remove your SIM card and reinsert it. Make sure it’s fitting properly, and the SIM card tray is closed completely. If possible, use another SIM card, to see it the problem persists.
You might also have to update your iPad, if it’s not running the most recent software. The latest iOS version for the iPad Pro (no matter which one) is 11.2.5.
It’s possible that there was a carrier settings update recently. Go to Settings - General - About. If there’s an update, you’ll get a popup which allows updating carrier settings.
Hope that helps.