My old iPhone still says locked to owner after removing it from my iCloud/Settings

I have recently bought a new iPhone so I would like to sell my old iPhone 12. I have been through the factory reset/ erase all content and settings and this is all fine. I have logged onto my iCloud account via a computer and removed the device from my iCloud. However, when I go to turn the iPhone 12 off/on it still says 'iPhone is locked to owner'. So I can't sell it.

I don't know what else to do as it has been removed from my iCloud so I can't see the phone anywhere anymore. I have logged back on to the phone with no settings/details added and tried to sort it from inside the phone itself but that isn't working either.

Any help/advice would be greatly appreciated.