Thank you for the response. I could not find an App called Sim Manager but eventually I found an app called 'Virgin' which turns out to be the same thing. I followed the instructions and selected 'international'.
I then went back to Settings, Connections, Mobile Networks, Network Operators, turned off Select Automatically selected T Mobile and it connected. I could then turn Select Automatically back on and it stayed connected. I suspect that restarting the phone after turning on International on the Virgin/Sim Manager App would be enough.
Thanks for the help. Very much appreciated.
Strange that this is necessary though.