@Andreipr 

In Access Point mode, Deco mesh runs its own WiFi network. It will not expand non-Deco router WiFi network.

Because of that:

Case 1 will work, and Deco X60s will form WiFi 6 network. You will have to turn off WiFi in the non-Deco router, otherwise you'll be running two independent WiFi networks, Deco mesh and non-Deco router, even if they broadcast with same SSID.

Case 2 is not going to work. Main Deco must be connected to the non-Deco router by Ethernet cable.

Andreipr wrote

 

So, case 1:  the advantage to turn off the non Deco router it's to be sure that the the devices will connect always to a wifi6 wireless network generated by Deco device, right? 

  @Andreipr 

Right. This, and more. See the answer on your next question for more detailed discussion.

In order to keep the existing wireless network coverage generated by the non Deco router I can leave it on. So, normally, in this case the devices that will be near from Deco access point will connect to it and the others, close from non Deco router will connect to old router without WiFi 6? (I will keep the same broadcast with same SSID).

 

This is correct. Think of Deco mesh as one powerful WiFi router. What you will have is same as deploying two WiFi routers at your place, both broadcasting with same SSID. It will work as you expect, with caveats. Mobile device may occasionally, on its own, switch from one router to another. Maybe device prefers WiFi6, or perhaps its preference is WiFi5, or due to some sudden interference the signal from router device is connected to became weak enough to trigger switch. That may happen even if device is stationary, such as Smart TV.

Whatever the reason is, when device switches from one WiFi router to another, connection interruption will happen. If you are on VoIP call at that time, it'll drop. If you are streaming video or music, that streaming will be interrupted. Netflix on Smart TV will pause, if TV decided to reconnect. File download will stop for a brief moment, maybe resume automatically but maybe not.

At my house, one of my family members often walks between rooms and floors with smartphone in her hand, while streaming music and video. Seamless smartphone transfer between WiFi mesh nodes is paramount in that case. I only run Deco mesh WiFi, my ISP router has its WiFi turned off.

How bad that would be at your place depends on specifics of WiFi coverage you'll have, and mobile device WiFi preferences often known to device software developers only.

You can try, if you get satisfactory results, so be it. Yet, if you'll get WiFi interruptions you don't like, just buy extra Deco node and attach it to router by short Ethernet cable, and turn off router WiFi.

Ideally, if you need to expand your X60s mesh, you should be getting another X60. If budget must be considered, for that specific purpose of adding Deco node instead of router WiFi, you can get the cheapest Deco X-series model, which is usually Deco X20. Attach Deco X20 to router by Ethernet cable, turn off router WiFi and you'll have true WiFi mesh at your house.

  @Alexandre. 

Thank you very much for your answer!