The USB WiFi dongle on the desktop talks to the router downstairs.
Your issue seems to be too far away, or there is too much stuff in the way (house construction, microwave, other electronics), or something else.
If it's just one floor in the house, it should work. As suggested, experiment with the orientation of the antenna on the router.
And of course, ensure the router wifi is actually on and broadcasting. Do you have a laptop you could test with?
Turn the laptop on, connect to the router, and walk upstairs, monitoring the connectivity as you go.
(and you're welcome)