Using my own router with BT Business Hub 5

We have just had BT Infinity Broadband installed. They installed a BT Business Hub 5. I want to attach my own router to this, because it is already set up to connect via VPN to our head office in USA. This is a Billion Bipac 7800DX. BT told me I have to enter the broadband username and password in the Billion device, and set the BT hub into Bridge mode. Can someone tell me how to set the BT Hub 5 into Bridge mode?

Thanks - Rowan