Pure Talk (which uses the AT&T network) sims work with the Librem 5. Calls, text messaging, data, and MMS/SMS all work. When you call to activate with a Customer Service Rep, they will ask for the IMEI from the modem and tell you that the phone isn’t compatible. IT WILL WORK!
You will have to contact Purism support to enable VoLTE and MMS/SMS. They will email you a site where you can download the script to run in the terminal that will produce an app on the phone to enable VoLTE. As for MMS/SMS, that can be enabled from the settings within the Chat app.