@Alexandre.

I am in a similar situation and having been doing self help by reading your past helpful postings. I am hitting a road block and required clarifications and assistance from you.

X20 WiFi Mesh constraint as a router is inability to set up 2 separate SSID (i.e. one for 2.4GHz and another for 5Ghz). A turnaround solution is to use Guest network as another separate SSID (https://community.tp-link.com/en/home/forum/topic/170006).

But in another post (https://community.tp-link.com/en/home/forum/topic/156175), it was mentioned as possible if I used Access Point mode.

I set it up as Access Point mode with a different SSID from my existing router due to network conflict issue.

Now I have 3 SSID - 2  from my router (2.4GHz and 5GHz) and 1 from X20 WiFi Mesh.

I fool heartedly believed that X20 is able to recognise and extend the existing WiFi coverage for 2.4GHz SSID of router without the need to connect to X20 SSID.

Apparently not since X20 runs its own network.

I am curious as to how does what was quoted in your previous post worked: "In Access Point mode, it is possible to turn off separation between Main and Guest networks, so that smartphone or tablet connected to Main network can manage IoT device connected to Guest network."

Also, does having 3 SSID triumphs over having one X20 SSID set up as router?

Sorry for the long post and questions.

Thank you for your patience. 

 @ymhun 

This is how I configured my home network with Deco mesh.

I have ISP router, with WiFi tirned off.

I run Deco mesh in Access Point mode.

Main Deco and two Satellite Deco are wired to ISP Router. One Satellite Deco wirelessly connected to Main Deco.

Deco mesh broadcasts two SSID: one is 5GHz for devices that can connect on WiFi5. Another SSID is 2.4GHz only, for IoT devices that can't handle WiFi5.

Here is screenshot from WiFi Settings in my Deco app:

I decided not to isolate Guest Network from Main Network, because at my household I have no other need for Guest Network but to have IoT devices connected to it.