Your guess is as good as mine. Have u not picked up SIMs from several different providers to test their signal? I have 2 main SIMs, one with virgin and one with O2. So in essence I'm using the EE and O2's networks. When one isn't any good, the other usually works.
It's definitely worth getting some free payg SIMs and finding one that serves you better than virgin are. Imo of course 😁
Sorry to hear you're having some issues with the signal in your area.
You may just be a little unlucky in terms of where our network partner's masts are located in relation to your property, however you can certainly take a look at the signal you should be receiving by popping your postcode into the signal checker here.