Welcome to the
Kitz forum.
Reading through you post, I sense that there might be a little confusion.
The Virgin device, ideally, should be in bridge mode just offering you Ethernet frames. The ZyXEL device should then be configured with its last (?) LAN port as an EWAN port, a pure router, etc, and the two devices linked with an Ethernet patch cable. However I suspect that the Virgin device cannot be configured as such.
As I am totally unfamiliar with the Virgin device, it might be best to wait for other members to comment. But my feeling is that you may have to have a "double NAT" set-up.