@Lubch 

Start with this FAQ article: Why my clients don't roam/connect to the nearest Deco node?

For technical details, also check this: Get to Know Fast Roaming / Seamless Roaming on a Deco System

The fix for laptop and any other device that can connect on 5GHz should be the following: using Connection Preference feature for that device, configure Preferred Band to be 5 GHz (from default Auto). Leave Preferred Deco at its default Auto setting.

By doing that you will ensure your laptop will roam much faster to the nearby Deco unit.

  @Alexandre. 

I already have configured preferred bandwidth as 5GHz for laptop.

And it connects to far away station, instead of neatest one. And laptop, and Deco mobile app mark connection as a poor, and speed is corresponded low.

Could it help to have main network as 5GHz only, and create Guest one for 2.4 GHz only? Or it has no matter?

Thanks.

Unfortunately, it doesn't work.

I've changed network configuration to 5GHz only, rebooted all Decos, removed all preferences which station to use and it happened again - my device (harman/kardon citation oasis) is connected to far away main station with mark in app about poor connection, instead of using 2 meters away station. That very sad, but Deco system doesn't work well 😟

  @Lubch 

If you install WiFi analyzer app on your smartphone, you will see what every device at your house sees about Deco mesh. Just walk to the place where you want to run measurements, turn off/on WiFi on smartphone to force it take fresh readings, and start WiFi analyzer app.

Here is an example from one of the places at my household, for 5GHz band:

There are three Deco nodes, and where these reading were taken Deco Node 1 is with strong WiFi signal, Node 2 with weak signal and Node 3 with almost non-existent very weak WiFi signal.

If I ask you which Deco Node the device that sees this should be connected to, you will for sure answer "to Deco Node 1." 

Now, if I ask you whose fault it is if device while having this info connects to, say, Deco Node 2, what would be your answer: A) Deco or B) Device?

  @Alexandre. 

How should I know which is fault is?

The fact is - I have 5 different devices with the same problem (Google Pixel 7, harman/kardon citation oasis, Acer laptop, Samsung TV qe65q80c, Deboot Ozmo 900 series). The reason could be any, but I've installed Mesh which doesn't work out of the box, how it's described on the market.

The fact is - devices connect to station with poor signal, and I'm not sure the issue is with totally different devices. It's Deco's issue, or configuration issue of these Deco stations.

  @Lubch 

Welcome to the world of WiFi mesh, where everything "just works seamlessly" according to marketing materials and in theory.

If you think this could be Deco mesh fault, you should return your Deco X50 set and try different brand. With Deco mesh you've tried everything you could with its configuration settings, and it still does not work for you.

Usually, in cases like this recommendations are:

1. Disable 2.4GHz band in Main Network settings for everyone or in Connection Preference for specific device. WiFi signal on 5GHz band travels shorter distances, which encourages device connect to nearby WiFi node;

2. Use Connection Preference to instruct non-movable device such as PC or security camera to always connect to nearby WiFi unit. This is not guaranteed, because device might not understand these instructions from Deco mesh or will outright ignore them.

3. Make sure "Fast Roaming" is enabled under Advanced settings. Note that this supposed to help with devices such as smartphones and laptops that move around the house. For non-movable device such as Internet radio this feature does not do anything: such device is expected to pick WiFi node with strongest signal and stay connected to it.

After you've tried all three, there is nothing else left but go with different WiFi mesh brand. This will give definite answer to the question "whose fault it is when device does not connect to nearby WiFi node?"

  @Alexandre. Thanks for your help.

Now I've reset all to factory settings, replaced main and second between each other (second station now became main) and configured all from scratch.

I can't return back, few months ago I've bought it. And usually Deco works fine. Hope it will work better after reset... Now all clients are connected to nearest station. Will see.

Step 3. Make sure "Fast Roaming" is enabled under Advanced settings - was disabled (by default I guess, I didn't change it), I've enabled it as you proposed.

Thanks