You need a very special router. The vast majority of router that say they have QoS have lots of fancy panels that mostly do nothing at all. You can set all the DSCP values you want and the ISP will set it all back to zero the moment they get the data.
Unless you are one of the rare people who are exceeding the upload bandwidth most routers that have QoS will do nothing at all. Sure you can limit which machine gets to send data first to the internet but seldom is the bottle neck traffic being sent it is traffic coming in.
They key feature you need is the ability to limit the download bandwidth to a fixed value and do this limitation based on the ip or mac addresses.
Most Asus and TPLINK routers have this feature as well as things like dd-wrt. Tomato firmware has one of the most advanced QoS but it take a lot of knowledge to setup in a way that really works.
The key problem is the ISP is in complete control of any traffic being sent to your house. Since they could care less what is most important to you they will randomly throw stuff away that exceeds the bandwidth you have purchased. There is nothing any router can do to recover this data so very technically there is no way to have QoS on the download link.
Still the feature that limits download data to fixed values for certain users does mostly work but for a completely different reason than QoS