Setting up QoS doesn't help...

Because it is all a bunch of lies

Well not really true but they do their very best to deceive you into thinking all those fancy menus with all those options actually have a use.

You can only truly control outbound..ie upload traffic. People seldom have a issue that they have exceeded their upload bandwidth. Someone with bit torrent or maybe someone transmitting a hi def video stream on a small internet connection could exceed the upload.

So if you have a upload issue some of those QoS things can have a effect but lots like DSCP markings and stuff mean absolutely nothing in a internet installation.

On download the ISP is in full control. When the bandwidth coming to your house is going to be exceeded and they need to drop or delay some data they will just randomly discard stuff. They have no idea....and don't care...what you think is important data or not they just throw away whatever will not fit at that moment.

Now when your router gets what is left what can it possibly do. It can't recreate the lost data you think is important and discards something else. So even if you would implement QoS on the download side it does not have much impact. Only the ISP can implement QoS before they send it and you would have to pay them to do it if they even offer the service.

Now what some people do is use the QoS features to try to place a hard limit on non important traffic. What this does is discard even more of the non important traffic after you get it. Of course this still does not recreate the lost important traffic. What it is dependent on is a error recovery feature in the tcpip stack of the end machines to slow the request rate down. So you are trying to trick the pc watching youtube into asking for less data.

This partially works but is not actually QoS that is doing the limitation it is dependent on the software being run. It does work but it is a very advanced configuration. You may be better off loading dd-wrt or even the asus wrt firmware if you even want to attempt this. It is not something simple to get to work consistently.

So for the router manufactures to try to let people think you can limit traffic just by clicking a few boxes are very close to telling lies.

I would Google and see what people say, but you should be able to revert back to the original Asus firmware.

See https://www.bestvpn.com/blog/10680/custom-router-firmware-in-depth-review/?nabe=6600234480173056:0&utm_referrer=https://www.google.com.au/

here's the tutorial I found to configure QOS properly and it worked for me.
https://www.youtube.com/watch?v=AssAlsDVgm0