OK, I give up.
I tried all sorts of different combinations but I can't find an Admin page for this model. I scanned all addresses from 192.168.0.0 to 192.168.255.255
I DID find that all four ports are bridged! There is a GigE port and three 10/100 ports labelled 1,2,3. All of these are in bridge mode, I have a dedicated IP, assigned to the MAC address of my Unix server, and all four ports will give my firewall the correct IP.
My conclusion is that this modem is nothing more than four bridged ports with no wireless, no DHCP, no DNS and no Admin page.
It's kind of nice that there isn't a lot of other junk that I don't need running but it's a little disconcerting that I'm locked out.