@commequidira 

My Deco mesh set is two M9 Plus and two M5s. Main Deco is M9 Plus. This is how devices connect to my Deco mesh just now:

It is not very busy place, but as you can see each Deco has devices connected to it.

The simplest solution would be to disable 2.4GHz on Main Network:

Devices that can only connect on 5GHz will roam much faster to the nearest Deco.

If that does not help or not feasible, more info will be needed from you: screenshot "Show Map" like one I shared, also "Show List" and "Update Deco" screenshots from Deco app, to start with.

  @Alexandre. 

Thanks Alexander, that's a great idea! I will try to do that! If I understand correctly, I place my Deco of the main network in 5Ghz but the secondaries remain in 2.4Ghz? Or I do this in 5Ghz and after a while I put my main Deco in the 2.4 and 5ghz?

  @commequidira 

This configuration setting is for all Deco units in your Deco mesh. They all will stop broadcasting 2.4GHz on Main Network SSID.

  @Alexandre. 

I just tried the experiment, unfortunately the devices did not connect to the other decos. I will look for other solutions. Thanks again, I appreciate it.