Wifey's iPhone 7 is on iOS 12.4 (yes needs updating). Intermittent cell signal past few days, got a system message that said phone needs to be updated to latest iOS. There's a known problem with some models of A1660 that the modems go bad...cell service has become suddenly intermittent the past 3 days, it would work if I removed SIM, powered down, replaced sim, then restarted. Now this doesn't work; it has no cell access at all.

Confirmed with Apple Support Tech, it's a covered issue. Genius Bar appt isn't till Thurs and my wife is gonna be very upset being cut off 'til then (what did we do 30 years ago, right?)

1) I wonder if an update will help or not because Settings->About->Modem is blank. I read in a comment online that means the modem is kaput. Can anyone here with knowledge confirm this is correct?

Is it worth a shot to update iOS anyway, maybe TMo eol'd tower accessibility for phones on iOS 12? Seems far fetched as it's not terribly old, but who knows.

2) Is there a way to go from iOS 12.4 to 13.6 without going all the way to 14? Is this the path, will it work and how to install from a download...it sounds like iTunes won't let it install since it's "unsigned"? [ipsw.me]

Hesitant to go to 14 because of all the problems that 14 users are reporting. Open to your thoughts. Thanks.