iPhone 12 Not Activating??

hi,

since no one’s offered any thoughts, i’ll give it a try.   

i haven't had the same problem, but i do have some professional/technical network architectural engineering experience, an iphone 12, model number MGG23LL/A, working on t-mobile's 5G network in north Seattle, WA.

i purchased the iphone online at Apple.com & activation was pretty much automatic.  as it was for my cellular+wifi ipad air model MYJ32LL/A.

i just installed the sim, previously in a motorola <something i forget> gsm android handset, and didn’t think further about it.

if i’d had your problem, i’d have hoofed it to a t-mobile kiosk and asked a retail employee to look at it.

if that failed,  i’d have tried a different sim, from a different carrier, maybe a “burner sim” from 7/11 or walmart, a  store like walgreens, rite-aid or similar.  failing these, i'd try to get an RMA from apple and send it back to them for repair/evaluation.  

good luck!  i figure you’re frustrated and i hope a little free advice and thoughts might help.

jon