@Mikis I have had to do the exact same thing with my X95's.  I honestly think it's just a deco wide issue.  You'll have some people that'll say "Oh it's up to the device to choose", which it's not.  The deco's should be able to route the traffic/devices to the strongest signal.  

I have devices in my office/game room that are literally sitting in the same room as my office deco and yet they want to try and connect to the living room one that's more than 40 ft away, I don't understand.

I'm hoping this newer firmware (1.11.0 / 1.5.0) they keep posting about will fix these kind of annoyances, it's just a waiting game on when they'll arive on certain devices with no eta unfortunately.  It's stated to have improved networking performance and stability.

  @Mikis It's definitely not up to the device to choose. That's my 45 years as a computer network engineer speaking. (But i could be wrong). Let me explain why.

A quick test will show if you look at the app, at the devices connected to which node, that if you unplug the satellite node, all of the devices jump immediately to the main router. (Refresh the app and assuming the device is in range to get "some" signal). There isn't even a blip on the device. I've done this test with a streaming security camera.

Also, the concept of a mesh system is a "reject/accept" communications between the main deco router and it's slave units. They are in constant communications with each other. Your devices are actually receiving the same frequency carrier from ALL of the deco units simultaneously. Just like the devices see other 2.4/5g wifi signals that you aren't logged into. The main deco determines based on the signal strength of the device which deco node it will send the requested data through. This is also why when the slave node is disconnected, the main unit simply send the data to the device without an interruption.

Where I think part of the problem lies, is with the deco's ability to properly measure the signal strength. For instance, right now, as I type this forum, I am in the next room from where the MAIN Deco router node is. My iphone shows MAX SIGNAL from the main router; yet the app says my iphone is connected to the slave node upstair and only has 2 bars. (I just came from upstairs). After refreshing a few times, the app finally showed that my iphone was connected to the main unit and had full max strength bars like my phone was showing. (Back to my earlier point that your device is communicating with ALL the deco nodes at the same time).

So it seems, that the deco software/firmware needs to be more accurate in communicating with the devices and more accurately measuring the signal strength that the device actually has. Then it will be able to better determine which node to send the data to. So in my recent case that I just mentioned, even though my phone was receiving max signal from the main router node, it was actually receiving it's data from the node upstairs because the software in the deco didn't change yet to the proper node.

For portable devices like my phones and tablets, I don't really care. I move around enough and as long as there's a connection, I'm ok with it. But many of the fixed devices if they are powered down or switch networks (Like to my VPN router wifi) and come back later to the deco, it doesn't always get the best connection. Not only does it not necessarily grab the best node, but it may grab the proper node and strong signal, but choose 2.4ghz instead 5ghz. If it's capable, it should always grab 5ghz.

I put a battery backup on my slave node, so this shouldn't be an issue that much any more with my fixed devices, but it shouldn't be like this.

  @Mikis 

If you power off a satellite Deco, all affected devices will, hopefully, promptly switch to another Deco.  The switch is expected to be transparent (not affecting the traffic), because the Deco mesh supports the IEEE802.11r protocol, making such a switch less disruptive than a regular switch from one access point to another access point.

A device is, at a given time, connected to a single Deco, and exchanging data with a single Deco.  Each Deco uses a unique wifi identifier, the BSSID, and each device is connected to a specific BSSID.  Even if several BSSID are using the same frequency, a device is, at a given time, only communicating with a single BSSID. 

  @Mikis 

You wrote this:

For instance, right now, as I type this forum, I am in the next room from where the MAIN Deco router node is. My iphone shows MAX SIGNAL from the main router; yet the app says my iphone is connected to the slave node upstair and only has 2 bars. (I just came from upstairs). After refreshing a few times, the app finally showed that my iphone was connected to the main unit and had full max strength bars like my phone was showing. (Back to my earlier point that your device is communicating with ALL the deco nodes at the same time).

If you iphone shows max signal from your main router, that means it is connected to the main router.

You certainly cannot trust the app to permanently show you the exact status.

  @yves_b  Not sure you understood my point. (Could definitely be my posting)

My main point is; the deco app does not accurately reflect the signal strength of the devices connected. As such, it will not always accurately switch the appropriate satellite nodes to device.

Example: Right now, again as I am typing, I am sitting on my front porch. I know I am connected to the deco node in my main living room. It is about 20 feet away (Through the front door of course). The deco app correctly says I'm connected to this node. The chances of my being connected to the main deco unit is pretty slim considering the reason I went with this mesh system is because this was a somewhat dead spot with my traditional wireless router. Now, having said that, my IPHONE shows MAXIMUM Bars and signal. It should.... after all, my iphone is only 20 feet away from the deco node through a door. However, while the deco app accurately says my Iphone is connected to the main living room node, it shows that the device is only receiving 2 out of 3 bars for a "Medium" signal. My Iphone shows MAX signal.

It is this inaccuracy of the app communicating with the main router, satellite nodes, and devices which is giving inaccurate information TO the main router; which in turn doesn't allow the main router to make good decisions communicating with it's satellite nodes as to WHICH node should be handling the traffic.

If the app accurately showed the signal strength that the device is showing, then when that device is relocated, it would see that a different node will provide a better signal. Also, it should also determine that a "2 BAR 5ghz" signal is BETTER than a "3 Bar 2.4ghz" signal. Because that is also another issue many people encounter; having their devices downgraded from 5ghz to 2.4ghz unnecessarily. Now, for basic internet traffic, 2.4ghz and 5ghz doesn't really matter which is being used. But if you are doing video conferencing, download/upload of files, and other data intensive processes, 5ghz definitely passes data faster, cleaner, and is affected less by interference. Especially if you are near the kitchen with a microwave working; baby monitor; and many other 2.4ghz devices.

The issue is a daisy chain. The deco doesn't always connect the device to the most efficient node. It doesn't always connect on the most efficient band. This is very noticeable if one of the nodes reboots or temporarily disconnects and none of the devices come back to that node when it's operational again. (Unless you manually disconnect and reconnect the device or manually tell the deco to connect the device to a specific node). And all of this is partially LINKED to the fact that the deco doesn't accurately know what the device has for a signal strength. This is not a difficult task. A simple Apple CarPlay and Android Auto displays the cell phone's signal strength, displays it battery charge, it's signal 4g, LTE, 5g, etc. The principle behind how the mesh router sees the devices is quite similar. It should have no problem accurately determining the signal strength. But the deco node should not be showing different parameters, like signal strength, than the device.