Can't reset email password

I have an old bellsouth.net email account. After changing phones, I've lost access to it.

When going to att.yahoo.com to log in, I try to reset password and it takes me to att.com. I try to do a forgot password. I put in the bellsouth.net email and I put in my last name. It gives me the following message:

Hmm… we can’t reset your password

We don’t have the account details needed to complete your request.

So I contacted att on the phone and they can access the details of it, but says there's no additional email linked to the bellsouth.net email so they don't have the option to reset it or something.

It makes no sense that they can't reset it for me if the att.com page isn't permitting me to even reset it. I can get into one of the old sub accounts I created that's also bellsouth.net, but not the main one.

Does anyone know what to do?