I have now 6 Deco M4s (size AC1200, Versions I have to confirm later.) at home to improve my network stability. I bought 3 last year and 3 this year. My house is not to big (about 3000 sqft) but has multi floors. My main internet line is a Fiber to home and it comes on the back side of the house. That is where the modem and the main deco is. However, next to that room I have my office room. My bed rooms are right above the room where the main Deco is and other rooms are farther away in the basement. 

I placed 2 Decos in the basement, one in the office room and one in the kitchen and one in close proximity of kitchen in the living area. When I am in the office room even though I am sitting almost next to the Deco my computer or mobile keeps dropping from the network and shows "connected without internet". I have set up two networks which is a main network and the other one is guest network. Both emitting both band WiFi. I am using the Decos as WiFi router and I shutdown the modem's WiFi. Same problem appears in the basement where I have an office and where one worker works from that office. Because of this problem I have to use wired connection to my and my staff's laptop, But we also have tablets that we have to use and those also keep disconnecting from the networks off and on even though my staff sits next to the deco in the basement as well. 

Not sure if my Deco's are having problem or my internet provider has the problem. And there is also another problem I noticed, even though I am sitting next to the Deco and it has good signal from the main deco, my computer or device picks up a distant Deco and connects to that and does not want to switch to the nearest one. I have Fast Roaming and Beam Forming settings on the app. I cannot separate the issue and I have checked my devices as well. My findings are below - 

1) I checked the speed in my modem almost everyday that i had disconnecting issue using my other devices or after I reconnected to the WiFi with the same device. My test shows 300 mbps down and 100 mbps up (that is what I have as service).

2) The devices that keep disconnecting are not older models but newer model with WiFi 6 handling ability. I do not have the same problem when I take them to my actual office network at a different location. 

3) While sometimes the devices connect to the WiFi at 2.4 ghz (even though I am next to the Deco) the connection speed is very low. I only get faster speed when I disconnect it from the WiFi and if reconnects back to 5 ghz.

4) The Decos I bought last year has version 1.5.4 Build 20220113 Rel. 38349. The Decos I bought this year now has version 1.6.0 Build 20210917 rel 45957. I was having significant problem when I added the new version Deco as non main deco. All Decos were losing connection and were getting disconnected. So I had to make one of the new Decos as a main Deco and older Decos are now connected to that Deco now. Even though there are 6 Deco's 5 of them draws connection from the main one and only one in the basement connects to the nearby Deco above that floor (living room). 

5) I tried to update the older Deco firmware to the new version, but it does not allow me. It seems like there is no new updates for the older Decos. Not sure why. 

6) I have moved around the Decos to see if the issues are related to placement and coverage, I have not seen any difference. The placement I have now is more logical as the areas I have the Decos are most trafficked area where I am or my staff walk around with our mobile or tablet where as the laptops are more stationary in one place mostly.

7) At this time all the Deco's are mostly having stable connection without getting disconnected from the Mesh network. Yet I continue to face disconnecting issue of some of my devices (I set them priority as well). 

8) there are some older devices at home which connects to 2.4 ghz network (such as TV, Google Chrome) and they are mostly stable. Normally 8-10 devices are connected to the network all the time but only 3/4 are active at any given time. When the kids are home they watch their TVs and Tablets which is when the heaviest load is on the network but the issues I experience are not necessarily at that time. It is really random and happens several time during the work hours.

I am really sorry for the long post, but having read many posts here I realize it is better to provide all information at once rather than in pieces which does not really solve problem but wastes people's time and energy when they are giving one or few suggestions and questions back with something else. I hope someone can take the time to read this long post and give me some ideas what is going on and how can I fix the annoying problem and how can I update the older Deco's to newer firmware version. Thank you in advance and lot of appreciation.