No access to ATT/Yahoo! mail Web page of all my AT&T sub-accounts' emails

I don't know when AT&T stopped supporting to AT&T sub-account which is linked to primary account.  AT&T mail log-in asks for user ID only (not email), so it will only get me to the primary email account's ATT/Yahoo! mail Web page.  Sub-account was never offered to assign an user ID, so they could only be signed in by email.

My email client program still can check emails from these sub-accounts, but I cannot find access to them on ATT/Yahoo! mail Webpage. Therefore, there is no way to change setting, blocking or forwarding etc. of these sub-accounts.  Buy the way, I find it is not possible to set up email client NEW CONNECTION to these  sub-accounts.  I feel  AT&T is cutting them out of support.  Is that right?  I need to know before it happens.

I need to change email settings.

Can anyone help me to find a way to log into these sub-account ATT/Yahoo! mail Web page from my PC?

Thank you very much.