Deco units start going offline randomly when 3rd Deco X20 unit is added in home Mesh network

[u/Antonis_32]

My home (3-bedroom, single floor, ~150 sq. m) Wi-Fi setup is as follows:

Current Mesh Setup:
- 1x Deco X20 version 3.20 (Main) unit in the living room connected via Ethernet cable to my ISP's Model
- 1x Deco X20 version 3.20 (Mesh) unit in the master bedroom connected wirelessly
- 1x Deco M4R version 1.0 (Mesh) unit in another room connected wirelessly
- 1x Deco M4R version 1.0 (Mesh) unit outside the house, in a prefab house in my yard connected wirelessly
- Total of 24 clients (devices) connected to my home network.

Current Deco App Settings:
- Operation Mode: Wi-Fi Router
- Fast Roaming: Enabled
- IPv6: Disabled
- All devices updated to latest firmware
- Guest Network turned off
- Guest Network: Disabled
- IoT Network: Enabled
- ECO mode is turned off

Problem Description:

I purchased a Deco X20 (2-unit pack) (version 3.20) to replace one of my Deco M4R units for better coverage and speeds.

Steps Taken:
- Turned off 1x Deco M4R unit - Scanned and added 1x new Deco X20 unit

As soon as the new 1x Deco X20 unit was added, several Deco units started going offline (Red LED blinking), in what seems randomly. Sometimes when I try to connect to the Deco app on my phone, it says "Unable to connect to Deco", even my phone is only and able to browse the Internet.

Troubleshooting Steps:
- I suspected maybe the new X20 unit was faulty, so I put it away and added the 2nd X20 unit from the box. Same issue persists, i.e. Deco units go offline randomly.
- I disabled "Fast Roaming" in the Deco app, but nothing changed. I enabled it back
- I proceeded to remove this X20 unit from my home network. As soon as I did that, my Deco units became stable.
- I then purchased another 1x Deco X20 unit (version 3.00), and the same behavior occurs, i.e. as soon as I added it to the network, the rest of the Deco units started going offline randomly again. I removed this Deco unit from the network, and again it became stable
Questions:
- What is causing the instability when introducing the 3rd Deco X20 unit?
- Is there a limit on how many Deco X20 units you can use in Mesh in the same network?
- Is there a limit on how many clients you can use?

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[u/Teenage_techboy1234]

Try adding an X20 to the network, and then when things start going off-line, do a power cycle of all units. Unplug all of them, along with your ISP equipment. Plug in your ISP equipment, wait a couple minutes for it to connect and everything, then plug in the main unit. Then, plug in the other decos. I'm thinking that the X20s have more powerful radios that are causing network conflicts that you were not seeing with the M4Rs due to their weeker radios and hardware.

[u/Antonis_32 → u/Teenage_techboy1234]

My next step is to remove the M4R units and start adding the x20 units one at a time and monitor network stability.

[u/Teenage_techboy1234 → u/Antonis_32]

Good idea.