Why does my phone say it is using AT&T 34.0 as my carrier yet says my AT&T SIM card is incompatible?

I just bought a used iPhone 7 plus, an upgrade from my old iPhone 6. I have checked many IMEI websites, however none of them have said anything about which network the phone is locked too. So I checked settings, and saw that the carrier is AT&T 34.0. This means that the phone is locked to AT&T right? So why isn't my AT&T SIM card, which had been working perfectly on my iPhone 6 not be a supported SIM card on the 7 plus?