When you plug the desktop into the router, this is by ethernet cable, right?
If you have the same problems when connecting via wireless, and via wired cable, it suggests the problem is probably on the desktop itself.
It might be possible that you have some other software on your PC using a huge amount of data. Assuming you are on Windows 10 (a safe assumption, imo) here's a simple way to see the programs that are using data on your PC:
https://www.windowscentral.com/how-m...ows-10-devices
Are any of the programs on your data usage details list surprising?