@nigel-tufnel APN settings will only matter for mobile data and MMS. If the phone does not register at all, thats a different issue then. I am afraid you need to find an American user for confirming that, however.
Hi thanks for the reply. Unfortunately I am a "weekend warrior", work commitments leave me with little time to tinker with it, but when I get some at weekends , I will try again. I have now some non fully working phones to test (bad motherboards etc) against and I will try another version of TWRP (I tried the latest) to see if that works better. I am just hoping it is not coincidence and either the RAM or (not CPU) eMMC has failed (I have access to a reflow machine, but with COVID I don't know when. Thanks for all your suggestions, when I have some spare cash I will start donating to UBports as I think it is a really worthwhile cause, anything to stop hardware going into a rubbish tip.