Causes network congestion on WiFi

I have a My Cloud Home, which I exclusively use as a base for my Plex server. Whenever the MC is connected to my router (Asus AC66U_BC) it causes total network congestion within minutes, even though my Router does not report the MC having any traffic. My WiFi speeds plummet from 90+/90+ Mbit to around 2/2 Mbit and most of my devices cannot access the network/internet, at all.
When I unplug the MC, speeds are returned within 3 seconds.
I have tried resetting the Router, enabling QoS and none of it helped.
When the MC is plugged in, there’s no problems streaming things to my TV, which is also wired into the router.

Any suggestions/solutions would be very welcome.

How are you measuring network congestion?
Are you using WD Discovery and Desktop Sync to the My Cloud Home?

Well, to clarify; I use on my phones and laptops, and they tell me the internet-speed on my WiFi plummets when the MC is connected. As for my LAN WiFi speed, I don’t actually think that is affected, but haven’t tested it on my portable devices. The weird thing is, only things connected to my WiFi, seem to be affected by the speed-drop. My two desktop PC’s, that are connected via cable, are not affected at all.
So it seems that the MC clogs up the internet on my WiFi, but not on cable.
And yes, I do use Discovery, though just as often I move the files directly onto the MC, having it mapped in Windows. Moving files to it work as a treat, even when the mentioned network problems occur.

did you managed to find a solution for this problem?
i have the exact same issue as it over utilize the ADSL router itself
i use ping test on the router from my laptop and it gives packet loss and high delay
once i unplug the WD cloud ethernet cable everything back to normal
i really have no idea whats happening here as this suddenly happened after 4 month of normal usage

Well, Yes and No! I only use the WD My Cloud for running a PLEX server, so it occured to me, that PLEX might be the problem. I was running the PLEX app that comes with the WD, and every time the WD had been without power, for any reason, it would cause massive Network congestion. I then decided to remove the PLEX app, and install PLEX on my home computer, using the WD as an externa drive, instead of using it as the actual server, and lo and behold, never had a problem since… so for my part the problem was the PLEX app, for the WD MyCloud.