@CHAACHO 

You can't disable SSID broadcast on a single Deco unit, but there may be other methods available for you to convince devices that are near the third unit to connect to third unit.

How far from second unit will you place third unit, which will be connected to second unit by Ethernet cable?

  @Alexandre. Thanks for answer.

Unfortunately, the idea is to use the third at few centimeters only from the second one and connect them by cable.

In my project, the idea is to use the second unit only as a wifi/cable relay and the third one as a wifi source for my wireless devices.

Something like this :

[Main Deco] WiFi [second Deco] Cable [third Deco] Wifi [wireless devices]

Maybe I can configure the third one separately as an access point only, but I'll loose the mesh feature.

Idea ?

  @CHAACHO 

I see what you are trying to do. It is creative. I have a feeling that won't work, meaning you won't get double speed from third unit, but I can't find good explanation, why.

You should try that, I am really curious what the result would be. 

With X50 you have connection preference feature, where you can configure device to connect to preferred Deco unit. Try that with devices you have, and run speed test from device which will connect to the third unit. 

If speed test really delivers double speed comparing to device connected to second unit, then you can start looking at options for how to convince devices to use third unit instead of second. 

  @Alexandre. Thanks again.

I did the following to try: I used only two Deco units and I connected a third party WiFi router in access point mode like this :

[Main Deco] WiFi [second Deco] Cable [access point] WiFi [wireless devices]

I configured a different SSID on the access point and connected the wireless devices to it.

Results on copying a big file from my wired NAS to my S22 Ultra using FTP:

In normal Deco mode (two Deco units connected with WiFi backhaul, and smartphone connected to the second unit WiFi) : 30 MB/s

Using a third party access point wired to second Deco unit : 70 MB/s !

I also tried to connect my desktop directly to the second Deco using Cat 6 Ethernet cable and I got 71 MB/s when transferring the same file !

So I think that my theory is good: Deco X50 can really act as a very good WiFi gateway if you connect to it by cable, because in this case you keep all the wireless bandwidth to do backhaul connexion !

So...any idea to use this configuration with 3 Deco units and keeping Mesh ?

  @CHAACHO 

Can you return this 3-unit Deco X50 set? If you can, I suggest replacing it with tri-band 2-unit Deco X90 or Deco X95 set. As a tri-band it'll have separate band for backhaul. Deco X90/95 is quite expensive, they are top of the line Deco for WiFi6, but you may be able to find 2-unit set for the price similar to 3-unit Deco X50 set.

If not, and you have to stay with X50s, I would suggest you start with assessing which devices will benefit from faster speeds. I mean, really benefit, not just show improvement in speed test. In a regular household, smartphones, tablets, network printer and perhaps even Smart TV would not.

Laptop, Desktop PC, NAS - these most likely would. From those, of course, whatever you can wire to Satellite Deco - you should wire.

What's left after that assessment could be 0 devices - then while your trick is a neat one I will definitely keep in mind, it won't be necessary.

Perhaps, you'll have small number of devices you can't wire to Satellite Deco that will need top speed they can get. For those, use "Preferred signal source for a client device" to configure them connect to unit 3. It is Part 2 of the following document: How to select a preferred signal source on the Deco App

If these are devices that play nice with Deco mesh, they'll stay connected to unit 3.

  @Alexandre. 

Thanks a lot for your answers.

Unfortunately I cannot return my X50 and replace it by another model, but it's not a big issue. I'll follow your advice and wire the devices that really need high speeds.

At least, it was really funny to do such tests !