Using a Tesltra Gateway Max modem on iiNET NBN

I have a C6300BD-1TLAUS cable modem, provided by Telstra (an ISP).  I want to switch over the the National Broadband Network (NBN), with a different provider.  With the NBN, the NBN Co provides a cable modem to the NBN with an Ethernet Output, which then feeds into the WAN input of the user's router.  I would like to re-use my Telstra Gateway Max (C6300BD-1TLAUS) as the router, expoiting its WAN port as the input (instead of the cable input).  However, when I try, my computer doesnt seem to be able to see a signal in the WAN port.  It seems to be looking to the cable input for its internet signal.  (It gives an error that there is no cable connected).

How do make it so my computer can see the WAN port, and its associated data?

Thanks.