Does that router have more than one "internal" port? Then all ports besides the one supposed to go to the internet are, in fact, a switch, and plugging the line from upstairs in one of the "normal" ports should do the trick too... without knowing any device types or seeing pictures, this is all guesswork though.

I'd say it's either one of the real time strat games like Age of Empires, or the old hack'n'slay thing Diablo II LoD...

Hehe, that's pretty much not the point I guess, but at least I'm going to post more than most others. So there. :D