@davdav 

With each new iteration of WiFi standard they deliver faster speeds but for shorter distances, and more sensitive to obstacles such as walls and ceilings. Which means, replacing WiFi5 mesh with WiFi6 will not deliver better signal to the top floor. Actual result will be either the same as now with WiFi5 or even worse.

Getting Ethernet cable to top floor is the best way to solve issues you are facing. You should do that. If you can connect Ethernet cable to the second Ethernet port of Main Deco, you will not need to reconfigure Deco mesh. Top floor Deco, when connected to Ethernet cable, should instantly recognize Ethernet link to Main Deco and use it.

The end result will be WiFi speeds from top floor Deco become same as WiFi speeds you are getting from Main Deco.

  @Alexandre. 
Thank you for the excellent explanation.

so I have to connect the eth cable to the second port of the main Deco (not to a free port on the modem) and then as soon as I connect the eth cable to the Deco on the top floor I should have the same signal quality as on the ground floor (where is the main Deco).
About wifi5 vs wifi6 discrepancy is clear, on 3 floors of the house I wouldn't improve, in fact considering the walls and stairs the wifi6 would have more difficulty in distributing the signal throughout the house

  @davdav 

You have to connect Ethernet cable to Main Deco, this is correct.

If there are no issues with Ethernet cable, as soon as both top floor Satellite Deco and Main Deco are connected through that Ethernet cable you'll see in Deco app top floor Satellite Deco reporting Ethernet link. In example below Bedroom Deco is connected by WiFi, Office and Study Deco use Ethernet cable:

If after you connected Main Deco and Satellite Deco with Ethernet cable Satellite still stays on WiFi link, that means something is wrong with the cable.

In case it'll be challenge to remove and replace Ethernet cable, I would recommend to test it beforehand. This would be the right test: bring top floor Satellite Deco to where Main Deco is and connect them both with Ethernet cable you plan to use. If Satellite established Ethernet link, you can now lay that cable through the house with the confidence it'll work.

  @Alexandre. 
thank you very much now I also understand that the ethernet icon appears in the list view whether the satellites are connected to ethernet or not.
I also see you have a mixed situation with one satellite in wifi and the other satellites in ethernet. for the cable test, a good idea, the only thing is that I have to pass the cable through the walls first so I can crimp it after passing it, maybe in the meantime I'll do the test with another cable just to see if I connect the satellite to the main Deco with cable and check if on the list view I immediately notice the ethernet icon

  @davdav 

Hi

Do you have notice that connecting an ETH cable to the second port of the main deco can freeze the connection? As everyday day I had a PS5 connected via ETH cable to the modem, I connected a second PS5 with the ETH cable on the second port of the main Deco, result: the first PS5 disconnected, and theon both PS5s were no longer able to connect to the internet. both by trying to connect them to the modem's ETH network and to the main decon wifi network and to the modem's wifi network itself. they no longer connected. I had to turn off the modem and deco and reset everything and everything worked again.

now i try it again. I connect the eth cable on the second port of main Deco,  In the first time it seems that all works good but after few seconds both ps5 connection dosn't work. i dont understand why the first ps5 dosnt works that it is connect via eth cable directly to the modem ?!?!?! If i connect the eth cable on the second port of the main Deco It works only the wifi modem connection.
If i connect both ps5 via eth cable on the modem both works good. Very very strange thing, there is a freeze on the connection when i connect the eth cable on the second port of the main Deco.

  @davdav 

I have Sony SmartTV connected to second Ethernet port of Main Deco, works just fine. Not sure what is going on in your network, but it is unusual.