I had this problem too. I had several bars, indication of LTE or 5G when in a network area, but my new iPhone 12 Pro wouldn't connect. I spent 2 hours with three different VERIZON techs all having me do the same things; reset networks to factory settings, turn airplane mode off and one, turn on roaming data, blah, blah, blah... Then is dawned on me what the problem was... NORTON 360!!! I tried resetting certain parameters and even deactivated everything, but that didn't fix it. The I DELETED NORTON 360 and IT WORKED!!! I did that and everything worked fine. The next day after I finally calmed down, I figured that since Norton 360 was transferred from my previous phone that didn't have 5G, perhaps if I reinstall Norton 360, it would still work because the settings would be automated based on the new iPhone 12. And YES, it worked!!!