iPhone 6 no cell service

Hi.   I had a iPhone 6.    sometime over Xmas 2018, I took a iOS upgrade to 12.1.2 and I immediately had all sorts of issues with my cellular service.  phone works fine on wifi, but cellular service became non existent.    - when I went to settings, cellular, I didn't have a network selection option to choose.   Reset network settings helped, temporarily, as did a full hard reset.  I also tried backing the upgrade to 12.1.2 out and reverting to 12.1.1.       I have read the reports of phone being bent, re-seating  the cellular antennae etc - I did all those things, and re-seating the antennae.  They all appeared to help.  Temporarily, but  24 hours in, my service vanished again.   Makes me think it was the turning off and turning on that helped temporarly, not the actual actions I took.  I use my phone for my business, so as i’d had the phone several years,  I decided it was time to bite the bullet and change it.

I got myself an iPhone 7, and 3 months in, I had the exact same symptoms - no coverage.    This time, there was no apparent catalyst - no iOS upgrade anywhere near it.  it hadnt been dropped or mishandled in any way 9as far as i can tell).  I was able to take this one back to Apple and they tell me its a known issue (but only with the iPhone 7) and its gone off to be fixed.    It occurs to me, its a very similar issue to the one my 6 had, so if apple can fix the 7, presumably I can fix the 6.

Does anyone know what this known issue is so I can fix my iPhone 6.   (my 6 has had all kinds of new (non original) parts (battery(x2) , screen, BT &  wifi antennae, but all many many months ago) , so there is no chance apple will look  at it.

Any ideas where I should look?

Thanks in advance.

Craig