For what it’s worth, it seems to be an issue that is hard to diagnose for whatever reason. I own a small company that designs and sells WiFi products. When we train new support people we really emphasize not to forget MAC address filtering as a potential problem because otherwise they forget about it.
I think it’s because 1) it’s an extra step that few people take to secure their network and when they do they tend to be experts who don’t forget they did it and never call in for support. So of the small pool who use filtering, an even smaller pool forget about it or had their network set up by someone who didn’t explain it to them and need help. And 2) it’s hard to distinguish from much more common problems like people forgetting what their network key is. The only thing that really made it an obvious possibility in your case was that the USB dongle worked when plugged into your computer.