Have you taken your Mini out of the house to see if other WiFi networks show up? Meeera’s suggestion is a good one, too.
I found a discussion on the AT&T forums where somebody’s router had disabled the 2.4Ghz band. Something called bandwidth steering may be causing the problem, though I don’t know if this is your issue. More details in that forum.
https://forums.att.com/conversations...d5f2f606df0f16
Update: I’m not sure how I missed it, but you already addressed band steering.