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.

WIFI off

2g/3g/4g connection type

APN stuff. Modify existing two and save

-Cricket Internet-
Used for
Internet
Name
Cricket Internet
APN
Endo (think this was the main fix from default ndo)

-Cricket MMS-
Used for
MMS
Name
Cricket MMS
APN
ndo
MMSC
http://mmsc.aiowireless.net
Proxy
192.168.196.78

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