Invalid security email for yahoo to send verification code

I have a yahoo email account that now has an invalid email address setup in the security for validating who I am - I never added a phone number since when I was an at&t subscribing customer - now that the at&t email and yahoo email are separate when I try to login into my yahoo account it requires a verification code and I have no other options to try and access my yahoo email (which I used up until Dec 28th - on the 28th it required me to renter my password which only put me to my at&t email which had none of my folders or emails prior to Dec 28th, 2017 or contacts since I was using yahoo.....none the less, calls to at&t have gotten me no where since I'm no longer a paying customer and yahoo help tells me since The email address I had supplied years ago is no longer in existenance that they cannot help me retrieve my email account.......help.......