Is your time and date correct? Is your GPS position correct? I've had this problem with other brand phones when one of these is wrong.
Also, I don't know what it's like to use AT&T's actual service. But I've used MVNOs on AT&T's network and found that some devices work on specific bands where only certain data plans work. So I recommend seeing if your SIM works correctly in a different device, and then check if it uses different RF bands than the PinePhone. (I don't know the first thing about the PinePhone so this might not even be an issue).
Also, I don't know what it's like to use AT&T's actual service. But I've used MVNOs on AT&T's network and found that some devices work on specific bands where only certain data plans work. So I recommend seeing if your SIM works correctly in a different device, and then check if it uses different RF bands than the PinePhone. (I don't know the first thing about the PinePhone so this might not even be an issue).
