@Colson Yes, check all decos and they are currently with firmware 1.4.2 Build 20200116 Rel. 46064

@PieroSimonassi Ugh, that sucks. Wish I had a suggestion. 

Seems to be perpetual bugs coming out with these firmware upgrades. If youre out of your return window from the place you bought it, youre kinda out of luck if you want your money back. I got sent a replacement deco m5 set and even that is buggy , I was told I cannot get a refund, they will only debug it again remotely. All this seems like a big waste of time for me now considering they are convinced this is due to wireless interference -____-.  

@PieroSimonassi Did you try turning off Fast Roaming? I had to do that to make one of my devices connect to the Deco.

Also, for the problematic devices, did you try turning off "Mesh" (Seamless Roaming), for those devices? In the deco app, under devices, toggle the Mesh support for the problem devices.

@Kevin_Z 

After many tests I believe I have found the problem, but not a good solution.

I will try to explain in as much detail as possible.

Forgive me for English, my native language is Portuguese.

My network is set up as follows.

ISP - Net Virtua
Bandwidth - 240mbs
ISP moden - Arris TG1692a configured in bridge mode
Router - Deco
Switch - Tplink TL-SG1008D

The Decos m5 are distributed in three environments (living room, office and master bedroom).

The main deco (room) supplies the switch which in turn supplies the second deco (office) and the third deco (master room) works only by wifi.

The distance between the main deco (room) and the deco (mater room) is not great (about 10 meters), however the 5ghz signal suffers with a lot of interference arriving at the room practically zeroed. (I am on the 11th floor and there are many wifi networks, more than 60 networks). To solve this problem I added the third deco (office) powered by the switch and with that I solved the 5ghz signal problem in the fourth.

In my house I have about 26 devices connected to the wifi, a large part (21) smarthome equipment. All devices work at 2.4ghz and have the mesh option manually disabled since they do not require roaming. The other equipment (laptops, desktops, TVs and cell phones) have the mesh option enabled.

The fast roaming option is enabled - “802.11 r”

I made sure that all equipment (laptops, desktops, TVs and cell phones) can work with 802.11k / v / r standards, but here is the problem.
 

The decos (main room) and (office) are relatively close and get into a fight over who will manage the client. And then the problems start, the wifi or wired communication between the device and the deco is stable, however the navigation starts to suffer intermittency and this happens even for the wired equipment.

Monitoring the deco through the web interface, when the problem occurs the CPU consumption is stuck at 99% for a few seconds and the navigation stops. After a few seconds I see that the decos are organized in the distribution of customers and navigation returns.

What tests were performed:

I disabled the mesh option for all clients and disabled fast roaming
Result: All customers connected to the main deco (room) rendering the other two units useless. The equipment that could work on the 5ghz frequency changed to 2.4ghz.

I disabled the Mesh option on all clients, but I turned on fast roaming.
Result: Some equipment connected to the other decos, but the biggest concentration is in the main Deco and I still have the problem for the equipment that could work at 5ghz and are operating at 2.4ghz

I activated the mesh and fast roaming option for customers who need mobility, and I deactivated the office Deco.
Result: The network seems to have stabilized, the distribution of customers was a little better, but I have no 5GHz signal in the room due to interference.

I activated the mesh and fast roaming option for customers who need mobility, and I made the Deco of the office as the main one and deactivated the deco of the Room.
Result: Network seems to have stabilized, the distribution of customers was a little better, but now I have no 5GHz signal in the kitchen, apartment entrance and laundry (more acceptable, as I don't need high speeds in these environments)

Well, this is my story and I hope it helps TPlink to develop some simple options that in my view could exist in the app deco.

Manually manage connected clients, being able to distribute them through Decos;
 

• In QoS, to create an option for IoT per client, we can thus limit the bandwidth for these equipments that normally travel very little data;
• Signal control by deco (increase or decrease the signal individually by deco) would eliminate the problem of disputing customers for decos installed nearby.
• 2.4 / 5 range control by deco (dedicating a deco unit to the 2.4ghz network since the 2.4ghz coverage is usually greater and is the most used for smarthome equipment);

And now it's waiting.