Yeeesss!!! Finally managed to sort out a solution without using the method outlined by Chrysalis in his thread but taking inspiration from his discovery of the Billion's bridge lan ip. (And the eventual discovery of the correct question to Google.
)
In the Asus's wan interface, there's an option to set up a number of services (up to 8). Service 1 is the default connection (how one connects to the internet), so I've set up a Service 2 with connection type being Ewan, then I've set it to static ip (I don't actually know that that's the right setting, it only seems right cos: a) every other option doesn't look right; and b) it works
), enabled the service, set Get wan ip automatically to No, then set the IP address to 192.168.2.253, Subnet mask to the usual 255.255.255.0, and lastly entered the Billion's bridge ip address as the default gateway. Et voila, as the Italians say, I can now access the Billion's GUI and, even better, use DSLStats to watch DLM give my line a good kicking, again.
Anyway, thanks to all, especially Weaver, for trying to help me, we'll see if my "Infinite Monkeys" solution holds up in the long term.
As for now, I'm off to give myself an infinite number of pats on the back...
PS If there's any inaccuracies in how I've presented my solution, please don't hesitate to correct those inaccuracies.
(Except for the Italians bit, that's my "sense of humour" at work.)
