Thank you for you post.
I apologise that is the case here - I do have to be honest with you, we will always try our best to make sure settings are right from this side and offer advice in terms of how to connect. But this isn't something we have control over when you're outside of the UK - which ever country you go to that's not the UK, we/you rely on the network in the country you are going to. It's something that is out of our control, there wouldn't be a fix to put into place unless the problem was over this side in terms of roaming not being active etc. We don't offer any guarantee that when you use your services outside of the UK it will work. I know it's not the answer you are looking for and for that, I apologise.
I can see from your previous post it has been advised there is not 2G or 3G service now in the USA and your phone would have to have VoLTE to make calls - is this the case? Can you let me know what handset you have please?