The phone is my prev phone which is a honor 9 so old but not too old. It worked perfectly fine until about 3 wks ago.
I have had the phone outside and it doesn't make too much difference.
I'm happy to do what I need to do to get this to work as I need to work from here which I can't do if not strong enough.
Well. the sim itself has no relevance. it's just the access token for the network.
the strength of the signal and the quality of the hardware are really the only things that matter here.. have you tried the sim in your new phone, see how it performs then?
if VM's signal isn't strong enough at your caravan, you need to investigate other carriers.
if your phone is the limiting factor (you could try the VM sim in your other phone?) then you need some different hardware..