How to log in to Apple ID on iPhone running iOS 6 (6, not 16!)

An elderly relative has an iPhone 3GS. It’s running iOS 6 — the latest version it can run.

I’ve been asked to try and get the 3GS syncing notes and photos to other devices.

At the moment the 3GS isn’t logged in to an Apple ID. I know the username and password, and those credentials are being used on an iPad and iPhone SE without any problem. The iPad and SE are both on the latest iOS/iPadOS 16.2.

How can I log in on the 3GS? I type in the username and password in Settings, which triggers the warning about someone trying to log in on the iPad. I allow it. Then the iPad shows a 6 digit security code — there’s nowhere to enter it on the 3GS. I’ve already entered the password on the 3GS.

I did manage somehow to end up at a screen on the 3GS asking me for a recovery key, starting with RK. I tried resetting the recovery key on the iPad, made a note of the number and entered it on the 3GS, but it didn’t work.

is anyone able to help?