Directly above and below a router (with its antennas pointed straight up/down) are where the signal strength is the weakest. If your router has multiple antennas, you may be able to correct this by tilting one of the antennas about 45 degrees. It's still not ideal, and it may cause problems for other people. But it's better than sitting in the holes directly above and below.
Phones and laptops have WiFi antennas. On phones it's usually internal and the length of the entire phone. On laptops it's two wires that sit inside the lid next to the screen. Most USB WiFi adapters on the other hand have tiny stub antennas the width of a USB port in order to keep the size small. These are next to useless as an antenna, and rely on overwhelming signal strength from the router to maintain a connection. In other words, they'll drop off in speed faster than phones and laptops as you get further away from the router, or if you're in the holes above and below the router. If there's a way for you to temporarily move your computer closer to the router (preferably on the same floor), you could do that to test if weak signal strength is indeed your problem.
There are a bunch of other factors which could be causing your problem, but based the info you've given the above two are the most likely causes I can think of. Try adjusting one antenna on the router. And if that doesn't work, I'd try replacing the USB WiFi adapter with one that has an antenna.