Sigh. Thought that worked. Now it's caught in an eternal validation loop.
And I have no idea wether or not I enabled two factor authentication. I'm sitting here with two MacBooks, two iPads, and three iPhones from different generations that are linked to three different AppleID's that have been being used for years. Too many variables for me to have clue about some setting I changed or didn't change somewhere along the line.
About the loop: the iPhone 4s is showing a message "Additional verification required", when I click settings it goes to the account screen waits a few seconds and then shows "terms and conditions". Then if I agree, it tries to verify and then gives the "Apple ID Verification Code Required" screen. But now doesn't send a message to the other devices. Then if I leave it for a second it gives the "Additional verification required" screen and the loop begins again.
Occasionally it crashes the settings screen entirely around about here.
And then 20 seconds later, starts up again.
Ready to throw it out the window.
I did a full restore with load from backup, but I think I now have to do a clean restore.
Great that it's secure, but there comes a point where you just want to scream at it "I'm me! For the love of all that's sacred, I'm me!"