After much fiddling and adjusting of router settings, power cycling, lead changing, rebooting and the rest, it's obvious that the Sure Signal won't work with the Netgear DGND 3700 router. It's almost certainly because the Sure Signal is designed to work with PPPoA networks where I'm on PPPoE. It's being returned tomorrow and I have to search for another solution. So if you have the same combination, don't waste your time - send it back!