OAuth 2.0 and Apple Mail on iOS 12.5.5

Last week I could no longer access my @sbcglobal.net email from the Mail app on my iPhone 6 (12.5.5). The app would not take my password even though I was able to login using that password on laptops and my iPad (also running 12.5.5). I was able to get to my email in Safari by going to mail.yahoo.com on the iPhone. But then that stopped working as well. Finally today I was able to login via Safari. However when I try to create a new account in Mail it always says either password incorrect (using Other), or "Something went wrong" (when picking Yahoo).

Things I've tried:

• restart iPhone
• delete Mail app
• re-install Mail app
• reset network settings
• add new Mail account via "Yahoo"
• add new Mail account via "Other"

IMAP settings are correct when using "Other". The issue seems to be when authentication is validated via signin.att.net. Could it be that ATT changed their servers so mail now requires OAuth 2.0 which the Mail app on my iPad supports but Mail on the iPhone does not?

Thanks for any help! I've tried looking for an alternative mail app but all seem to require iOS 13 (which I cannot update to on my iPhone 6).