I am afraid I can not help you with this. In my case it was a problem of my mobile provider. The SIM-Card was old (2017) and my provider stated that sometimes with the old SIM-cards there is a problem which occures after restarts e.g. related to updates/upgrades. I can not follow the comment of my provider, but however they sent me a new simcard and since then the problem was solved. I had no issues since then.
In your case I have no idea as the loosing of the mobile connection can be a result of many different issues so I am afraid you have to engage with the help-line of your provider.
Mee to, solved by resetting APN ( i think it was. )
Phone: Fairphone 3
MMS did work in the old /e/ development version 0.11 something i installed october(?) 2020 and phone have been running nicely as my main phone all time except a minor issue so i held off updating until last week.
After update to latest official, i could not surf without wifi, and MMS never worked receive nor send.
I have let it be for a week, including a few reboot and tries. Never a mobile data connection.
I tried to check if APN settings are correct, but in settings there is nothing at all listed under APN.
And there is a text saying this user can not access these settings! Why cant they even be shown??
Anyhow i went through settings and mobile data off and on, set preffered type to LTE (was already, let it again search for network and selected hallon, then went to APN and selected (the three dots in vertical row top right) to reset to default, and rebooted the phone.
Now it works.
AND: now under APN there is content:
3 SE data tre se
3 SE MMS data tre se
Maybe we should rise a bug about initialising APN?
My slow brain now remember an endevaour years ago when i changed phone, phone maker, and provider, and the new one first worked a couple days but then APN etc got misconfigured - i guess it synced down settings from old to new phone erroneously, used same phone number and (i think) google account.
So i wonder if maybe there is incompatible settings format from old dev version to the new version, and they got synced.
Do the APN setting page display a message in grey tiny text that your user do not have access rights to the settings? When i see that text, Mobile data do not work, no setting is visible and it cant make new. I have not yet figured out when that happens.
Have you checked you are not logged in as another user or guest user? I have no other nor guest enabled.
Job profile enabled? Does not seem to matter for me.
Sandboxing like shelter? Does not seem to matter for me.