I get my HBO free through mobile phone on AT&T, too.
When I set up AT&T TV, I used a separate username/password than my main AT&T login, and had all sorts of issues getting it to authenticate with HBO GO properly, but somehow got it to do so. It seemed to be a problem with cached authentication tokens or something.
Anyway, I managed to merge my AT&T logins into one (login via att.com and go through your profile to find the merge accounts link), and things have been simple since then. HTH!
Also, Make sure you're using HBO GO and not HBO NOW.
I’ll have to check out that merge thing. I have one with an email, one without.