After a week of frustration I discovered this discussion. My issue is different but I think it's related. And after reading the 31-page discussion I can confirm my suspicion that the problem I am facing is due to iOS 14.X.
My issue is that I switched from Mint to Hello Mobile last week with two lines (one on iPhone X running ios 14.3, and the other on iPhone 6s latest iOS 14.4). They both worked flawless on Mint before the switch. On the night I switched, i was facing two issues.
iPhone 6s only connected Hello Mobile 3G (not LTE), and only voice/text works but no data. (and Carrier Settings update in About repeated beng offered even I allowed it to update, so it seems the carrier settins never got updated). And, the APN in Cellular settings in all blank. With manually entered APN info (sas instructed by Hello Mobile Senior support), it was still a no go.
iPhone X has a bigger issue. At the moment I inserted the new sim, it pops up saying Activation Required. And I proceeded and the activation never succeeded (even prompted to connect to iTunes on a Mac, which also failed to activate). There is one user in this discussion also mentioned his/her phone is bricked (being asked to activate too after upgrading to 14. it's also iPhone X). Once I took out the sim, i can start using my phone (just no cellular service). The phone is a unlock GSM phone even though Apple suspected it was a locked phone initially.
and both sims work as expected if I put them on another iPhone 6s and 6) that runs 13.X and 12.X respectively. Both connected to Hello Mobile LTE.
Hello Mobile would not be able to figure out, while Apple is working on my case (for iphone 6s) with senior support staff (and thru their engineers), but they won't get back to me for another few days.
So, I ended up with two non-working lines after switching to Hello Mobile. (the number portings were successfully).