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.