What model Belkin you working with Steven?
Sounds to me like you need to configure your Belkin router to have your Wanadoo username and password (PPPoA or PPPoE) within it.
There may be an option within your Wanadoo dsl modem to do either transparent bridging or un-numbered mode. Not sure how to word this, but doing either of these things would shut off most all capabilities of the modem (dhcp/NAT) and just allow the ip address you get from Wanadoo to go all the way through the modem to your router, therefore letting your router do the private addresssing.
Have you tested the modem, by just plugging a pc into it to see if you get internet? I'd imagine if you did that, default settings would would give your pc a 192.168.x.x address. Your Belkin router would have conflicts with this, if it also is issuing 192.168.x.x addresses. This is what would be considered double NAT ... which can be done, but is somewhat difficult to configure.
So this is why I suggest finding out if you can set up your dsl modem to act as a transparent bridge or use un-numbered mode to allow your ISP issued ip address to reach your Belkin router. Hope this makes sense.
***edit*** If I remember right, in un-numbered mode, your Wanadoo username and password is still stored on your dsl modem, therefore wouldnt need to be configured in your Belkin router. Transparent bridging ... you need to configure your Belkin router with Wanadoo's PPPoA or PPPoE username and password. So ... guess the best way would be to go with un-numbered mode or your modem's equivalent.