My 12 wouldn't work with my old SIM at all. Had to "activate" the "5G" SIM that came with my Verizon iPhone. Older plan and the rep said I would have to upgrade plan to have 5G service (nope, 4 lines on family plan - would cost a couple hundred dollars a year more just for that). It did show 5G on the screen, however speed wasn't any faster. Could care less about speed anyway, so I set the phone on LTE only.