@Vlad-Nirky I've looked at a number of used phone sales sites and am unable to find a Xiaomi Redmi Note 9, that is in reasonable condition, that I could use here in the USA.

I have a Pixel 3a on the dev channel with Red Pocket GSMA in the USA.

MMS works both in and out ONLY when the WiFi is ON.

If I turn the WiFi off, I can't receive MMS, but I can send it.

I've tested this a number of times. I don't know if it is just intermittent or really depends on WiFi. I will try sometime when I'm away from home so that the WiFi is on but I'm not getting data over it.

Another data point is that I have an old T-Mobile SIM for which the account is closed. I used to use it on my Pixel 3a. It doesn't connect, but it does fill in the T-Mobile APN correctly.

Does this mean that my N9P is NOT carrier locked? That it will work with T-Mobile if I get a new SIM card?