@Bonhard 

I have seen that occasionally, but not always with one of my Satellite Deco M9: when there are no wireless devices connected to it, M9 does not even show standard heading. I think all it says is something like "Deco operates normally." Unfortunately, I haven't saved screenshot of that.

So, the question is: are there wireless devices connected to Dining Room Deco, or there are none?

You can check if you have devices connected to your Dining Room Deco, by drilling into info for each of your wireless devices in Deco app. In Overview screen, expand list of Network Devices, touch device names one by one and in their next screen touch cogwheel icon in the top right corner. It'll get you to the screen similar to this:

In this example, my Roku device is connected to Office Deco by WiFi, on 5GHz band.

If you can find wireless devices connected to your Dining Room Deco, what you have might be a reporting issue in Deco app for "Connected Clients." Unusual, but possible.

If there are no wireless devices connected to your Dining Room Deco, the question will be: is it because they prefer to connect elsewhere, or is it because you have defective Deco unit that does not broadcast WiFi signal.

There are ways to find if your Dining Room Deco broadcasts WiFi signal or not, and I can help you with that if you need assistance, but just checking which wireless clients connected where is fast and easy first step I suggest you perform.

@Alexandre. Thanks so much for the quick response. I have done as you suggested, and some of my devices are showing Linked Deo as "Unavailable for current firmware", with the Connection Type as "Main Network" but with no frequency in brackets, so I think these are the devices which are connected to the only Mains Plugged Deco3 I have in the hall upstairs. It actually looks like nothing is connecting to the Dining Room Deco M9, but the Study Deco shows it's signal source as the Dining Room. I don't get it. Any ideas?

@Bonhard 

Something is obviously wrong with that Deco unit. 

Can you clarify: do you run Deco mesh in Router mode (default) or in Access Point mode? Also, is Dining Room Deco wired with Ethernet cable into your home network?

This is just to have better understanding of your home network topology, which mey be useful in troubleshooting.

Meanwhile, I would suggest basic steps that any Tech Support will:

1. Power off your Deco mesh, starting with Satellite units, and Main Deco last. After that, restart your ISP router, by powering it off for at least 1-2 minutes, then power on. 

I was once surprised to find that bouncing ISP router fixes some Deco reporting issues. For example, in screenshot I provided Roku IP address listed as UNKNOWN, it was fixed after ISP router restart. Just Decos restart didn't fix that.

2. After ISP router is on and operational, power back on Decos, starting with Main Deco, and Satellite Decos after Main Deco got green LED.

See if that fixes issue.

If not, next steps would be to find if Dining Room Deco broadcasts WiFi signal at all, if removing it from Deco mesh and adding back fixes the problem, if swapping it with Main Deco fixes the problem, or if upgrading it with recently released 1.5 firmware will fix it.

These next steps I can give you in detail, but I'll need you provide answer on my questions and to power off/on ISP router and Decos.

@Alexandre. Thanks, I will try all this first. I run the system in Router mode, and the Dining Room Deco is WiFi using the Main Deco as the Signal Source (which is Ethernet connected to the Modem).

@Alexandre. Hi, I did as you suggested, using the power on/off routine exactly as you have described it, and it all seems to be working as expected now. Let's see what the week brings. Meanwhile, thanks so much for taking the time to reply, sincerely appreciated.

I would be very interesting to know how you would suggest checking if the Deco is emitting a WiFi signal. Also when I check for a formware update on the App, it tells me that I am fully up-to-date at V1.4.3 Build 20200921 Rel. 42500.

Thanks.

@Bonhard 

Glad to hear the issue was fixed.

>how you would suggest checking if the Deco is emitting a WiFi signal. 

For that, I would have told you to install WiFi analyzer app on smartphone. I use WiFiman from Ubiquiti. It is useful tool to troubleshoot WiFi. It will list SSIDs of WiFi networks visible in your location, and under each SSID it will list all nodes broadcasting.

For example, I have three unit Deco mesh with Main Network running on 5GHz only. This is what I see in WiFi analyzer under my network SSID:

I've hidden under "Deco Node 1-3" labels MAC addresses of my Deco units.

So, for troubleshooting the plan would have been to install WiFi analyzer, walk to problematic Deco node, turn WiFi off/on on smartphone, check if that node is visible in WiFi analyzer, with strongest signal. If Deco node is not visible while standing 1-2 meters away from it, that would mean it does not emit WiFi, something must be wrong with its antennas. 

Note that if you have both 2.4GHz and 5GHz enabled on Main Network, the number of entries will double as each node will emit two different WiFi signals. 

>when I check for a firmware update on the App, it tells me that I am fully up-to-date at V1.4.3 Build 20200921 Rel. 42500.

TP-Link released 1.5 firmware for Deco M9, but not everyone could get it through Deco app yet. They are doing gradual rollout. New firmware can be downloaded from Support web page of TP-Link Web site and installed manually.

As long as you are happy with your Deco mesh performance, and whatever new features they mention in release notes do not interest you, I would recommend just wait till new firmware is available in Deco app. Let someone else try it first.