Our daughter uses VM a bit further away, and has moaned recently about "only one bar". A few new masts have gone up these past few months just south of the village, but I have no idea who uses them.
If you pose your question on the community facebook page (Llandaff North & Whitchurch Daily Life Cardiff), you'll get responses I'm sure.
But expect some non-tech-savvy answers as people on there still complain about VM "turning the wifi off".
https://www.facebook.com/groups/954644197903602