I was able to resolve the issue of MMS/Group messages. Before anything else, I'm on Verizon, bought the iPhone 12 Pro Max through iUP through Apple. Also, doing these steps would reset all your settings including apps and data apps. You would have to install back all the apps you desire. Also to note, I also have Google Fi on my iPhone 12 Pro, and I used the same process, and it started to work as well. Same goes with my wife's iPhone 12 Pro Max.
Here's what I did to make it work.
- Checked to make sure the iCloud backup includes Messages, which is found in Settings -> iCloud -> Messages (This should be green to make sure all your messages are backed up.
- Forced the iCloud backup three times to make sure all messages are back
- Erase the iPhone 12 Pro Max in Settings --> General --> Reset --> Erase All Content and Settings.
- Erase all cellular plans as well. This would remove all MMS settings that were carried over from the backup (I think).
- When reset is done, set the iPhone up as new, and sign in with your iCloud account. DO NOT RESTORE FROM iCLOUD BACKUP. I did this twice and got me back to square one.
- Install all the apps you need.
- To recover all iMessages, go to Settings --> your iCloud account --> iCloud, and initially, Messages would not be checked. Enable the option (set to green). This would eventually recover all the messages you have saved through the iCloud backup.
- To monitor if you're not losing messages, install the Message+ app and configure (only for Verizon).
Let me know if you have any questions.