New Apple ID & Password, but iPhone won't allow see Apple ID change. Account keeps locking

Problem: iPhone wants old Apple ID password to sign into iCloud with old Apple ID.

I successfully changed my Apple ID and Password on Mac desktop computer.

I can sign into appleid.apple.com with new credentials BUT a banner shows "Do you want to sign in to appleid.apple.com with your (my old email Apple ID)? or "use a different Apple ID". The new AppleID and new password work.

Using the old AppleID and old password LOCK the account.

I click "FORGOT PASSWORD?" > I enter NEW Apple ID > I enter cellphone number to get a code sent to NEW APPLE ID email > code was received & entered > Credit Card info entered > "How to Reset Your Password Immediately" instructions screen appear,

I go to SETTINGS> MY NAME > PASSWORD & SECURITY> Change password> When I enter the new password, the account shows as LOCKED.

I have done the UNLOCKING steps a number of times to no avail: I'm stuck in a loop.

How to I get the iPhone to forget old APPLE ID and old password (whatever it was)?

Thank you.