Seeing my modem's webpage?

I'm a bit stuck. I have long used a Vigor modem and Linksys routers. I can't work out how to look past the router to see the modem (linestats and such). I can plug a computer straight into the modem and that's fine. I can even change its IP address to suit. But the router doesn't seem to see it. I've tried various things and nothing seems to work.

Should I turn off the DHCP on the modem? Would that fix it?

I think I'm being a bit thick here! I've put up with it for years now but surely it's fixable?

These days I'm using a Vigor 130 and it seems to be pretty much the same as my old 120. The routers have been various EA series.