Couple of things to point you in the right direction, and for others following along...
The carrier of your camera and phone do not have to match. The camera has its own service and can send to any phone on any carrier. some send via text, some via email, some do both. The important part is what service you have at the property where it will be used.
Poor cell service on your phone does not necessarily mean the camera will not work. Ive had cameras get 4 bars where my cell had 1. Im not a cellular genius so I cant explain it, just know it happens.
In your case where you have good AT&T signal, the Snyper should be a good choice. They will run SIM card from AT&T, Tmobile, or Sim Hero. Unless you have an unlimited plan from AT&T and plan to add the camera as a line, the Simhero is likely the most cost effective route.
All brands of cell cameras I have worked with have quirks in getting them setup but once they are up and running they are fairly trouble free. I generally help my customers get a cell plan going and setup/ test the camera before shipping to make it easier on them.
No matter what brand, use an external power source and solar panel. They eat batteries!
I dont get up that way very often. If you're willing to cover the shipping, I'll send you one to try.