handogg

you will know what to look for.

Att is an American Company and has NO TOWERS in Mexico what so ever!  SO..  You will ONLY pick up signal close to the border with ATT.  Just bleed off from the towers in the US..  So If you set your phone only to connect to ATT, once you pass into TJ more than a few blocks , you will more than likely be constanly searching for a signal.   Telcel is ATT roaming partner, as ATT is Telcels in the US, so that why they switch over.  

So, some things aren't possible and this one is one of them..  If you want ATT, you cannot use it in mexico, Because, as per ATT/Telcel Agreements, once you cross the border, you become Telcels customer so they can charge high rates, and the same in reverse for telcel customers going into att territory!

Oh, and as OB said, there are carriers that seem to get good reception over here, but they all seem to be CDMA , NOT GSM!!  
Sprint, Verizon, Cricket, and Virgin Mobile, are all CDMA technology and I guess the towers are stronger or closer, because all those seem to get US SIGNALS all over TJ,
but ATT and T MOBILE (GSM Technology ) seem not to work once crossed into Mexico.