I just popped in the nano-SIM card I had on my previous Android phone. Cell calls and data worked. In the Settings app > Mobile, I did have to toggle “Mobile Data” to turn that on. I found US T-Mobile settings for data and MMS and used those settings with one exception: I chose “Network Mode” as “3G, 4G (Preferred)” because there is no 2G on T-Mobile as far as I can tell, and they gave me a new nano-SIM card a couple of months ago because they said they were going to be dumping 3G. So I might try “4G Only” to see how that goes, but I did notice “3G” appeared on the status bar once.
I also had to edit the settings in Chatty so that MMS works, though when I get an image, it does not show up inline in Chatty. Instead there’s a link to where it got downloaded to a buried folder in ~/.local/share/chatty