@TP-Link 

1. Speediest.net gave me 10/10. Some times it can't start the test, I have to turn wifi off and then on ( both deco and my smartphone) and then begins to test.

2. My ISP provided bandwidth is 100/10, I never go under 90/8.

3. I didn't notice red led on deco.

4. Yes my Samsung S20FE dropped the wireless connection, had to turn wifi off/on using deco app, switch off 5/2.4 and turn it back on. Then the wireless come up again .

5. All wireless devices show no internet and some disconnect from wifi .

6. Yesterday I've switched to Access point mode (no luck) and then switched back to router mode with PPPoE. Also I've switched off GUEST SSID for both bands. I didn't change the main deco, i kept the previous one.

There is no specific time range for the connection drop 

@TP-Link 

1. Fast Roaming was always OFF. Beamforming can be disabled but only for testing, Beamforming is a must and was working with previous firmware so i can not disable it for long time.

2. DNS was always changed to 1.1.1.1/1.0.0.1. Tried 8.8.8.8/8.8.4.4 also. Same results or worse (larger ping time)

A. ISP is WIND Hellas

B. Provider modem is Technicolor TG789v2.

C. Network Topology is like: ISP Modem/Router (wifi off) Ethernet Cable from Port 1 to M4R (main) on port 1, Satellite M4 Wireless

D. No wired Laptops but i can add something

E. 40 Devices, 20 IOT (smart Lights, Smart Switches, Air-condition , Dehumidifier) and 20 Network Devices (NAS, Switches, Powerlines, SMARTTV,  MAC, PC, ANDROID Box) All the IOT devices are working @ 2.4GHz. 

As i write this , the speed is back to normal 100/10 and there is not a single connection drop (up and running at least 5 hours). When 10/10 occured i had to turn off and on wifi on the DECO using my Cellular Connection. No reboot. I am afraid doing now (it's working and i am working too!)

--UPDATE-- I found that Fast Roaming was enabled (i must enabled it in the morning when i tried to find a solution for the problems). Disabling Fast Roaming made the problem come up (Speedtest.net dropped to 50/9 from 100/10 earlier and wifi started to disconnect ). Enabled Fast roaming and after Optimising Network from Deco App the speed is back to normal. It's a temporary solution because not all my wireless devices support this feature .

I will try to set it up as requested. The problem is with the 20 Networked devices (not IOT) because at least 10 of them are using ethernet through the satellite M4R (I use the satellite Ethernet ports to connect a switch and a PS5). If this is not a problem i will have 10 device connected to the Guest 5GHz network.

@TP-Link 

Did some tests but not those requested 

1. Could not change to AccessPoint and test because when I did I lost internet connection so I switched back.

2. Disabling 2.4GHz and FastRoaming and leaving 5GHz with beamforming made the 2.4GHz devices to loose connection , 5GHz connect and disconnect in about one minute cycle. I switched off my smartphone wifi and connected to deco using cellular.  After 5-6 tries I 've managed to connect but after about one minute I lost connection to deco , the same cycle as internal network. Then after reconnecting I quickly switched on 2.4GHz and switched off 5GHz. Wifi connection restored. 

3. After switching off 5GHz and enabling 2.4GHz every IOT device restored connection with deco except two smart plugs . Switched off beamforming  and the smart plugs connected again.

4. Finally using my cellular connection to deco I enabled 5GHz and switched from cellular to wifi. My smartphone connected to 5GHz with no problems . Speed is restored both for Wan and Wi-Fi. 

All the steps above was taken without rebooting or changing the DECO devices.

@TP-Link Hello,

I have a problem after i update the deco m4r v2 system to 1.5.0 , the wan port no longer works at 1gbps, it works at 100mbps. If I plug the ISP cable directly into their router it works in 1gbps. If I connect a laptop to the LAN port of the deco system it goes in 1gbps, it no longer manages to negotiate in the gigabit port.

I also installed the firmware update recommended by you on the previous page and in vain . I disabled also fast roaming and beam foarming.

What should i do?

Thanks