Spark HG531S V1 - Bridge Mode/AP Mode

Anyone know how or if its even possible to set the HG315S V1 Modem/Router to act as an AP only.

i.e ADSL Internet ----> ADSL Modem ---> HG315 ---> WiFi Clients

I can change the IP address of the HG315 and disable the DHCP server but I can't see or understand how to make one of the LAN ports act as a WAN port so I can connect it to the ADSL modem. The ADSL modem will be the DHCP server.

Anyone know how I can set a LAN port on the HG315 to act as WAN and direct the WAN to the modem? Starting to think its not possible..

I can do this with my Airport Express easily, just give it a static ip address, disable DHCP and use the modem IP (192.168.1.1) as the gateway/DNS