I notice that the appartment is two floors above the router.
Because of the way the radio signal is emitted from the antenna, almost none of it goes straight up if the antenna is vertical. (One trick to visualise where most of the signal goes is to imagine a donut threaded onto the antenna. No signal at the ends of the antenna, but all around the sides, good signal.)
This means that you must point the side of the antenna towards the direction in which you want to have the best signal. In this case, that would mean setting up the antenna so it's almost parallel to the ground.
This will work with any router which has a positionable antenna. Plus, it's free!
A note about multiple antennas: place the antennas at 90 degrees from eachother, in a "V" shape, to get the best overall coverage, since each antenna will be covering the other's "dead zone".
I hope this helps :-)