I am preparing to change my cellular provider from AT&T
As some of you may know, ATT does not currently support the Librem 5 (although SMS is still partially supported):
I am currently using AT&T with my Librem 5. I just got my phone back from Purism service, and when I put the SIM card into my Librem 5, it connected to the AT&T network. I had to go into settings and reenable the APN (Access Point Name) of NXTGENPHONE.
I got a SMS text from AT&T saying my phone wasn’t supported, and I lost cell data. Restarting the phone got me back to 4G. Then the SMS → no data happened again. This time I went to my AT&T account and activated the Librem 5 (listed as unknown). (IIRC I had to click on SIM under the phone picture and number.) This made the issue go away.
FWIW, I had started on AweSim using both my Librem 5 with its own number and Android on AT&T. After two weeks, I dropped AweSim and my Android and put the Librem 5 under my old number on AT&T. Now that AweSim allows keeping your number I’m likely switching back to it.