The title is quite self-explanatory.

I have a brand new Archer C9 no more than 3 months old, everything worked, yesterday i built a PC for my little brother, he likes gaming and so do i. Everything worked great at first, but when he started updating/downloading gaming, he takes all broadband, 250 Mbps. 

So i enabled QoS, got his Mac address, and sat him in lowest priority 5% of the network, at first i thought that could solve the problem, it did nothing, so i added my PC as well to the highest 50% priority and our home media device to medium 25% (So far 80%) Still nothing.

So then i changed our Upload/download bandwidth to half of what we have, just in case we get less broadband during peak hours, from 225 Mbps (i took 10% off initially) down to 90 Mbps, still, nothing, whenever he updates/downloads he lags the whole network.
I know its lagging, cuz when i ping our router, instead, the typical 1ms respond i get 40ms to 200 something ms.

Another thing i see happen when he is updating/downloading, both our wired connections disappear from Status > Wired Clientes. Yet i can still see both computers under traffic statistics.

The last variable to the pie, he sits behind a TP-Link power line (AV600). It's not possible atm to connect him directly to the router, due to his room been quite away from it.
I have tried changing the QoS it comes with from "Internet" to "Online Gaming" and vice versa and nothing still lags.

So I'm out of ideas, i was looking for some old Bandwidth control, where i could regulate the max and minimum each client could have, but apparently C9 doesn't have it?  
Did i buy the wrong router?