@Adex1 

It is possible that Satellite Deco disconnected briefly, and that forced device connect to Main Deco. Poweline link is not the most reliable one. Is that Satellite Deco online now and does it use powerline link?

With Deco P9, one can't explicitly configure device to connect to specific Deco. That feature comes in firmware version 1.6, which P9 may never get. Still, there may be other options available to convince device to connect to nearby Deco. These options depend on device and its capabilities, so if you can tell me what that device is I may be able to help you.

Thank you so much for your reply  @Alexandre. 

Is that Satellite Deco online now and does it use powerline link?

Yes, the Satellite is online and currently using PLC/2.4GHz/5GHz

so if you can tell me what that device is I may be able to help you.

The device having troubles are a few but the one I really care about is my Apple laptop

  @Adex1 

For Apple laptop, I can recommend two options:

1. Configure Main Deco to run on 5GHz band only, that should provide more incentive for laptop to select nearby Deco.

2. Connect Apple laptop to Satellite P9 with Ethernet cable, you said the distance between them is very short. In case your laptop does not have Ethernet port, Apple Support FAQ recommends the following: "If your computer doesn’t have an Ethernet port, try using a USB to Ethernet adapter, or a Thunderbolt to Gigabit Ethernet adapter."

  @Alexandre. 

Unfortunately I cannot set the main Deco to 5GHz as the robotic vacuum and the Netatmo Valves for my radiators do not support 5GHz. 

Yes I am afraid that the ethernet way is the only one available. I hate to do that as recently this Deco started chirping and having it on my desk for 8+ hours will make me crazy (the issue shouldn't be due to the chirping as swapping the decos doesn't fix the issue). 

I'll proceed with the ethernet way for now, thank you very much.

If I could go back in time I would never bought these TPlink products, I hate when things no longer work when there's no change to the setup.

  @Adex1 

Another option you could try is enable Guest network, configure it with 5GHz band only and connect laptop to Guest network. Just to check if my theory is right and laptop connected to 5GHz will prefer nearby Deco unit. 

I am not very familiar with Apple laptops, but on many Windows laptops you can configure WiFi network adapter to stay on 5GHz exclusively. If test with Guest network gives expected results but you need laptop on Main network, you could research if it is possible to have laptop configured to stay on 5GHz - and connect it back to Main network.

The chirping sound for Deco P9 hardware V1 had been discussed on this forum. Can't recall exact details and haven't saved link to discussion, but it has to do with P9 hardware, capacitor or something like that. The only way to fix it is to replace that capacitor. If your P9s out of warranty - you are out of luck. Unless you are good with soldering, someone posted exact steps on how to replace that faulty component. Sorry, I don't have link to that post either.

@Alexandre. thank you for the suggestion, but if I check the WiFi connection in my laptop it shows already connected as 5GHz. In this moment the bars are all full the speed test isn't that bad but the latency is absurd, if I simply perform a ping to google.com I have 127ms via WiFi and an average of 25ms via ethernet 

About the chirping, I just teared down the Deco and ordered two capacitors, let's see how it will go

  @Adex1 

This screenshot tells me your laptop is currently connected to the Deco unit nearby. I could only get that excellent signal level on 5GHz when standing right in front my Deco. You won't get this quality signal on 5GHz WiFi from remote Deco unit.

Still, at some point of time your laptop may switch again to faraway Deco unit, for example if you bounce Deco nearby, and might not be rushing to reconnect to nearby unit. Hardwiring laptop to Deco is the best solution, but if you want I can give you a little bit more info on what is WiFi mesh (any brand, not just Deco) and mobile device role in device roaming between WiFi nodes. Which will give background for my recommendation to force laptop always stay on 5GHz and also some pointers for Apple laptop WiFi roaming settings tuning.

In regard to latency, 127ms is indeed terrible, but can you add a little bit more info to "25ms via ethernet?" Do you mean it is 25ms when you hardwire laptop to Satellite Deco P9, or do you get 25ms when you connect laptop to ISP modem/router by Ethernet cable, bypassing Deco mesh? 

  @Alexandre. 

It could be handy if TPLink allowed to ban a MAC address from the 2.4GHz band. While it's indeed a nice workaround, it sounds crazy to use a guest network 5GHz only as I wouldn't benefit for the Mesh technology that I paid for.

The alternative of creating a guest network 2.4GHz only for the devices that don't move (as sonoff, NETATMO valves etc) doesn't work either as I wouldn't be able to control those from the devices that go in the standard network that I would set to 5GHz...

can you add a little bit more info to "25ms via ethernet?"

That is the ping response when my laptop is connected via ethernet to the satellite Deco

 Like, right now I'm in my bedroom which is the closest room to the satellite Deco, why is the RSSI so bad?

I'm about 3 meters away from the Deco and in between there's just a plasterboard wall and a wardrobe

Also, why does it say Weak if it's connected via PLC?

  @Adex1 

The feature to force device on a specific band and to connect to specific Deco is available for newer Deco models with never firmware. It is described in How to select a preferred signal source on the Deco App, scroll to Part 2 of it. 

I doubt firmware with that feature will be released for P9. P9 is very behind in firmware releases already, last update was 3 years ago.

I can't tell why you have such bad latency on WiFi, I thought it is powerline issues but apparently not. When I ping google.com on smartphone connected to my Satellite Deco, latency is 20-30ms with occasional spikes to 50ms. My Deco runs on recent firmware 1.6, while P9 is on firmware from 2020, I believe. That may explain different experience.