It is likely that they linked over wifi.

Your BT router will have had a name it transmitted (SSID)
The new Virgin router will have a new name. You will have probably have linked your Mac to it using an encryption key (WPA)
The printer also needs to learn the new Virgin SSID and WPA key.

As mentioned a total reset of the printer, leading to a fresh setup where you enter these details virgin router wifi details is likely to be best.

More than happy to help if you want to DM me.

Make certain that you delete the current printer config on the Mac before you start the setup process so that it does have to learn it again.

Can you share the printer model ?

Everyone loves a Morgan. Even me, unless it's broken again.