SOLUTION TL;DR: Open and close every bloody app on new iPhone 12 and notifications will start.
SOLUTION: Hello All. I can't be sure that there aren't DIFFERENT problems that we are experiencing, but I had this exact problem and I fixed it. FYI calls to Apple and TMobile did NOT provide a solution.
WHAT WORKED: Having done a backup on my IPhone 6s, I then turned on my iPhone 12 (let's call it Classic) and used the blue cloud thingy to get one to talk to the other and downloaded my 6s backup to my new 12 phone. I got phone calling and text messaging to work and, interestingly, all my voicemails DID come over and did NOT delete. All my apps came over as well BUT no notifications showed up.
To get notifications from Apps after confirming that YES notifications were turned ON for all apps, I had to OPEN AND CLOSE EACH APP INDIVIDUALLY and then slide up HARD from the very bottom of the screen to the TOP which will give you access to all the still open apps, and slide each screen up to make sure each app is actually CLOSED. (As you know just tapping the screen to "close" the app off the screen does NOT CLOSE THE APP. ) A few apps that required log-ins did require me to log in to the app to make sure it was me and then notifications ensued. In many cases the passwords had already been saved in my old phone and the new one gave me access to my saved passwords, often after checking my FACE ID.
I went to bed and woke up to a slew of notifications.
NOTE: at this point I had NOT signed out of the Apple ID or deleted anything on my old phone, so what I did was to put them side by side and see what notifications were on my OLD phone and NOT on the new. This let me then go in to those apps and reopen and close or sign in and hopefully those will now show me notifications from now on.
This absolutely worked for me is all I can say.
Alley