Have you looked through your manual? There's a lot of information there, and at least several different settings that may help you.
The manual also tells you what QoS is (Quality of Service)
But, QoS is a feature of the router to make your network work more smoothly by prioritizing different types of connections. If your roommate is a gamer, or uses VOIP, etc. then your roommate with get priority for the bandwidth. The QoS engine is _off_ by default, so you could try turning it on, and see if that makes a difference for your situation. That setting is covered in the manual
Perhaps a better choice is to limit the time that your roommate can be connected - which is also a setting covered in the manual, in Access Control settings.
Or, maybe a simple "Get off the internet, Sam! I need to do some things, and you're sucking the pipe dry!"
Or - pay for more bandwidth from your ISP (get your roommate to help with that, too!)