I'm confused as to what you're connecting to - Virgin Cable or Virgin ADSL.
The WNDR3700 is suitable for their "Cable" service, while the DG834 range are for "ADSL" via the phone line.
If you are connecting the WNDR3700 to their Cable service, its just a matter of connecting its WAN port by ethernet cable to the provided Cable Modem or Set-Top Box. However, you should power off the Cable Modem for a few minutes so that it releases the IP address allocated to the existing connection (recognised by its MAC address). It will then recognise a new connection and allocate a different IP address to it, when you should get online.