My wife’s iPhone XR on iOS 14.3 was having the same issue.
After days of troubleshooting on my own, I finally got T-Mobile on chat and they fixed it.
It was the APN setting. You will also need to turn on MMS in Messaging once you’re done with these steps.
Steps
1. Select settings
2. Select Mobile Data
3. Select Mobile Data Network
4. Scroll to and select Reset Settings
5. Select Reset
6. Select Settings again
7. Select Mobile Data
8. Select Mobile Data Network
9. Enter MMS information
Here's the MMS information for T-Mobile. Google for other carriers info.
APN: fast.t-mobile.com
Username:
Password:
MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc
MMS Proxy:
MMS Max Message Size: 1048576
MMS UA Prof URL: http://www.apple.com/mms/uaprof.rdf