Pixel 3a XL here on development channel. Cricket wireless. After mixing and matching multiple apn solutions to get mms working, I figured I'd share my fix. Hopefully it'll save someone the heartache I just went through, haha.
2g/3g/4g connection type
APN stuff. Modify existing two and save
Endo (think this was the main fix from default ndo)
I believe changing the Cricket Internet to Endo helped with a conflict of both mms and internet being stock set to ndo? Not sure, but that was the last thing I tried before giving up, and it started working.
Hope that helps