If you’ve ‘lost’ your recovery key ‘RK-‘
To log into an old iPad where it asks you for a recovery key eg in iCloud, you need to append a valid verification code from another of your trusted devices (eg iphone/iPad with two step verification onto the END of your current password to bypass the RK- prompt. Took me a few searches to find so I’ll post here...
On iphone/trusted device with iOS 9 or above go to settings, tap name at very top, tap password & security & tap get verification code (six digit) and note it down. It lasts 10 min.
In old device eg iPad click settings, iCloud, account and in the password field, enter your current valid AppleID password and immediately after 6 digit code eg Hello123456 without any spaces. Tap done. Three ticks should appear next to fields and you’ll be logged on, without the RK- prompt!