Progress! I got up, toyed around on the new machine a bit and then decided to give the Allied Data Technologies modem another try, upload the 5.15 firmware again etc. The firmware itself went OK, but then suddenly something went wrong with the profile or so it seems. Which is truely bizarre as that same profile uploaded without problems yesterday.
At least I now have confirmation that the modem is truely dead : it won't even detect a lan connection anymore, nor does it react to the softkey which would tell it to return to default mode. This time I really killed it I think. Ah well... too bad because it certainly is a good modem with tons of features and options. I may contact ADT Holland and see if I can get it repaired for not too much money, but I'll go out and pick up a new cheapo adsl modem for the time being, so I can (hopefully) get this wireless stuff working after all.
I should really have kept KISS and LITHA in mind before I started working on it. Always remember : don't fix it if it ain't broken.