Something like that happened to my Tunetalk recently. It turns out that it's AT&T restricting the type of phones allowed onto their network. I researched for a long time and during one reboot, I noticed I hooked onto AT&T for just a split second. So that told me the SIM was working but AT&T kicked it out after that.

What I did was to buy a phone on the approved list: https://www.att.com/idpassets/images/suppor...ATT-Network.pdf , and Tunetalk SIM card immediately worked.