Just got my Red Pocket SIM (AT&T Network) and was running my initial tests on my Pixel 3a.
Sending and receiving calls works flawlessly! as well as the Cellular data. Issue seems to be receiving SMS/MMS messages. In my testing I attempted to send an MMS message and received an error about improper APN settings preventing the receive. I reset my APN settings per Red Pocket's provided settings for the AT&T network. I can now send SMS messages (sent many of them), and MMS as well, but receive nothing. Any suggestions as to troubleshooting steps I may be able to try? I am a long term Linux user so any command line debugging steps would be welcome.

--Update 54 minutes later one of my SMS test messages came through