Cookies must be enabled in TB Options/Privacy for OAuth2, and while TB supports OAuth2 for Yahoo accounts on imap.mail.yahoo.com, pop.mail.yahoo.com and smtp.mail.yahoo.com, I wasn't aware it would work with the att.net servers, even though AT&T mail is based on Yahoo. In that case, the usual recommendation is to use a secure mail key in place of the account password, and apply 'normal password' authentication instead of OAuth2. Is your 'Oauth password from AT&T' a secure mail key?

If you can get OAuth2 authentication to work with the att.net servers, just enter the regular account password in the OAuth browser window, and then a long oauth token will be stored in the TB password manager.

Cookies must be enabled in TB Options/Privacy for OAuth2, and while TB supports OAuth2 for Yahoo accounts on imap.mail.yahoo.com, pop.mail.yahoo.com and smtp.mail.yahoo.com, I wasn't aware it would work with the att.net servers, even though AT&T mail is based on Yahoo. In that case, the usual recommendation is to use a [https://www.att.com/support/article/email-support/KM1240308 secure mail key] in place of the account password, and apply 'normal password' authentication instead of OAuth2. Is your 'Oauth password from AT&T' a secure mail key? If you can get OAuth2 authentication to work with the att.net servers, just enter the regular account password in the OAuth browser window, and then a long oauth token will be stored in the TB password manager.