@BramS 

Can you share a bit more information about you home network:

Do you run Deco mesh in Router or in Access Point mode?

Do Satellite Deco units connect to Main Deco wirelessly, or do you have them wired with Ethernet cable?

Do you keep WiFi running on your ISP router after you deployed Deco M5? If you did not turn off ISP router WiFi, does it use same SSID as Deco?

What are these "WiFi devices?" 

When "WiFi devices" hop off nearest Deco they are connected to, do they all leave Satellite Deco and it reports no connected devices, or just some leave and some stay?

When you reboot one of these "WiFi devices," does it connect to the nearest Deco?

@Alexandre. 

Hi! It's al configured in Access Point mode.

The main deco is wired to the modem/router. Wifi in the router itself is disabled. The other two deco's are wireless connected and placed on the first and second floor.

The devices that jump around are the thermostat, 3 lightbulbs, Google home , Google nest, and 3 wireless power plugs.

They alle stay connected , there a no reports of signal-loss.

If I reboot any device, sometimes it connects to the nearest deco, sometimes it connects to the one farest away. There is just no telling.

For example, my cellphone just connected to the deco on the second floor, while the primairy deco is only 2 meters away. Rebooted the cellphone and it still connects to the second floor with poor signal.

The thermostat was connected this morning to the second floor and now it's connected to the first floor, even when the mesh-function is disabled in the settings.

I have firmware 1.5.7, build 20210819 release 43499.

I tried to upgrade to the beta 1.6.0. but that does nog work.

I hope this is enough information. If you need more I will be happy to provide it to you. :-)

@BramS 

So, Main Deco in the basement near modem/router, Satellite Deco on the first floor and Satellite Deco on the second floor, correct?

I have Deco mesh with M5 on firmware 1.5.7. I have Ecobee thermostat, a number of TP-Link KASA smart plugs, smartphones and tablets among other WiFi devices. Usually, thermostat and smart plugs are connected to the nearest M5. They jump to a different M5 when "their" M5 is rebooted or loses connection with the rest of Deco mesh. When that happens, they rarely come back on their own, staying connected where they are.

Mind you, I haven't disabled MESH on any of them. Fast roaming is enabled, too.

For these "WiFi devices" I would recommend you investigate the possibility of Deco M5 losing power (is it plugged well?) or losing connection to the rest of the Deco mesh. For each Satellite Deco in Deco app check what it says under Signal Source, example:

If you see less than two blue bars, yellow/red bars, no 5GHz and only 2.4GHz - this Satellite Deco is too far from the Deco it is connected to. That Satellite Deco may briefly lose connection to Deco mesh, and when it happens - your devices connected to it will jump elsewhere.

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For smartphones, tablets and other devices than really need high speed WiFi connection, the problem may be slightly different. Depending on the size of your house, smartphone may be getting strong WiFi signal from multiple WiFi mesh nodes and assumes it does not matter which WiFi node to connect to. 

I had that problem in my house, so I disabled 2.4GHz on my Deco Main Network. The benefit is two-fold: 5GHz signal gives faster speed, and it is weaker - convincing smartphone to connect to the nearest WiFi node with stronger signal. Smartphone no longer sees multiple WiFi nodes with similarly good signal.

Because devices such as smart plugs I have can't use 5GHz, I've enabled Guest network with 2.4GHz only, and connected them to Guest network. This is my WiFi configuration in Deco app, try that:

@Alexandre. 

}}}So, Main Deco in the basement near modem/router, Satellite Deco on the first floor and Satellite Deco on the second floor, correct?{{{

That is Correct😊

Thanks for the tips. I will try them.

Dutch Helpdesk from tp-link adviced to disable the mesh-function for devices that "do not move around the house" , like the thermostat..well, clearly that does not work. 😂😂

The signal source could be the problem, since the Deco on the second floor also "jumps" from the deco on the first floor to the one in the basement every now and then.

@BramS 

Make these WiFi configuration changes I recommended, they will improve your home network. Yet, the biggest problem as far as I can tell is your 2nd floor Satellite Deco jumping between Main Deco and 1st floor Satellite Deco. When you manage to fix that, your home WiFi mesh and network will be much more stable.

You said you tried firmware 1.6 upgrade, perhaps you can try it again? You only need Satellite Deco on 2nd floor upgraded with firmware 1.6, then it'll have an option to where connect and you should connect it to Satellite Deco on 1st floor.

Another option is to make Deco on 1st floor your Main Deco, but that will require Ethernet cable going to 1st floor.

Also, if you can connect Satellite Decos on 1st and 2nd floor with Ethernet cable, that will also fix that problem.