HELP Can't Access my old phone now I have transferred data to new phone via icloud backup

My wife bought a new Iphone and transferred her data ,by backing up her old phone to icloud, and then restoring phone via icloud backup on her new phone.

Now her old phone asks for a passcode to unlock the phone but her passcode does not unlock the phone.

How can I factory reset her old phone when I cannot access settings?

I cannot understand why her passcode wont work, is it because now the appleID is on her new phone the old phone becomes unaccessible? Surely I should be able to factor reset? Is there a way without passcode?