U.K. to USA iPhone issues

Moved as a family from the U.K. to the USA.

3 unlocked iPhones between (all bought in the U.K. all bought with the knowledge that we’d be moving to the usa). So we have a 6, 7 & an 8.

we are all now on Verizon. Have had no issues with the 7 & 8....just put the sim in and we were good to go.

but have had major issues with the 6. It will not recognise the network AT ALL. Have spent hours on line with the support reps and myself trying to sort it. The sim itself doesn’t appear to be the issue (it works fine when I put it in another handset).

I have tried everything (& I mean everything) to try to get this to work, including downloading APN profiles.

I am at my wits end. Short of getting another handset, does anyone have any suggestions?