Cannot see Broadband modem on LAN

Hello
this isn't for an ADSL installation - I have Tooway satellite broadband (I'm miles from the exchange), but the principles are the same - connection to the internet is made via a Viasat Modem with an ethernet connection.

I have a D-LInk DIR-655 wireless router.
As usual, it has an IP address of 192.168.0.1.
Connecting two or three pc's to the network works fine (using the LAN ports on the router) - one of the PC's is a laptop, connected via the wifi.
Everything works normally.

But I cannot get the network to see the Tooway satellite modem, and therefore the internet.

I've tried connecting the modem via a LAN port and the WAN port (on the Router), but with no success.
All PC's are set up for automatic IP address and DHCP is enabled.

If the modem is connected directly to a PC, excluding the network in other words, it works perfectly.

I feel I must be making a very basic error - can anyone assist and point me in the right direction please?

Cheers
Sean