I'm puzzled by your question. Looking at the Billion 8800NL page and then selecting the Application Diagram tab to show the rear of the device, I do not see any socket marked WAN. Reading left to right, I see: DSL LAN4 LAN3 LAN2 LAN1.You will definitely need to configure the device into bridge mode but I'm sure there will be people to assist -- if you get stuck.
This is a better link http://www.billion.uk.com/edu/VDSL2/index.html
The instructions are headed 2) PPPoE Bridged mode (FTTC / VDSL)