It was locked to the Boost Mobile network (Sprint) before I supposedly unlocked it with Odin, but using the ATT SIM card says it's not supported for the device. I've spoken to both companies and neither seems to be able to help. Thanks for any advice
There's a difference between unlocking a phone and rooting a phone. What ODIN can do is root a phone (giving you unrestricted access to the features and settings), what you need to do is unlock the phone (allowing you to change the cellular settings).
Once that's done, you need to set the cellular settings for the AT&T network (they would currently be set to the BOOST network). AT&T technical support is who should be able to help you with that, but the phone needs to be unlocked first.