ATT/Yahoo internal redirect at sign in

It's been a week and no email access.  At sign in - the user name and password is recognised and accepted.  No error message, no denial of access.  The 'sign in' screen redirects 4-5 times.  Including: "oidc.idp.???  , "sign in", "sign out", ATT.Yahoo.Att.net,  and more - redirect is fast, other addresses are missed.  After hours and multiple agents -  Att support - I'm told  to call Yahoo, "work it out with them".  With Yahoo - I'm told it's an Att problem because the email address is sbcglobal.net.   Attempts to login get the same result - Incognito, Chrome, Firefox, ATT.com, Yahoo.com, Safari.  Apple has accessed the device - viewed this sign in process and the redirect.  Apple identifies it as an ATT/Yahoo problem.  HELP!