@Alexandre. 

I have done all of these steps except powering off the 2nd node so what ya aim or expect from powering it off 

Many thanks for ur reply and help

  @DrAhmed86 

I want to be sure your Pixel connects to hardwired Main Deco and does not go through WiFi connected Satellite Deco. This is the simplest way to ensure it.

Also, trying different speed test systems - if they yield very different results, suspect becomes speed test system. That's why I suggested OOkla app and fast.com site in browser. If they do provide similar results, about 10%-20% difference is OK, then it is not speed test server that misleads you.

Other than that, I am running out of ideas. I am getting better speeds on my Deco WiFi5 running in AP mode (M9 Plus and M5, all hardwired) than you do from superior by technical and WiFi specs Deco model.

I also use Pixel 7 Pro for speed tests.