|StuartS has the simple, easy, logical, sensible solution.|
Move the router upstairs and connect the cable running between the basement and the first floor from the modem downstairs into the router upstairs. Since it's a wireless router, you don't need a WAP and with the router on your 1st floor, wireless clients on the 2nd floor should be able to connect to your WLAN without issue.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.