@RadekB 

The easiest would be to undo what you have done, and start again. Steps:

Delete all Deco nodes, one by one, from both Deco networks. When you delete Deco node, it will be reset.

Delete both Deco Networks. See How to delete a network on the Deco app

Create new Deco Network.

Add M4 as Main Deco to that Deco Network.

Configure Deco Network to run in Access Point mode. Verify your Main Deco M4 is operational after that change.

Add remaining Deco M4s to that Deco Network.

Done.

  @Alexandre. I have done all this several times with no success - still the same result. As soon as I switch 1st Deco do AP, devices on wifi periodically gain and loose wifi, Deco going red - white - red - white... both wifi A on Asus router and wifi B on Deco stops providing internet access.

  @RadekB 

Please confirm:

1. You have deleted (reset) all Deco nodes from Deco network, by following this FAQ, section "Reset a Deco unit:" https://www.tp-link.com/us/support/faq/1457/

2. After that, you have deleted Deco network from Deco app, by following this FAQ, section "Scenario 2:" https://www.tp-link.com/us/support/faq/3353/

3. You have powered off all Deco nodes, thus removing Deco mesh from your home network.

4. After that, with only ASUS router, you have WiFi coverage from ASUS, devices do connect to the Internet, and connection speed is good when devices are in near proximity of ASUS.

If you haven't done that, please get your home network to that state. Obviously, what you have now is not working. 

Also, on a side note, ASUS router should not be impacted by Deco mesh, but if you use same SSID for both your WiFi networks, ASUS and Deco, that could cause some issues. Do not use same SSID for both WiFi networks.

  @Alexandre. 

Solved. But first things first - yes, I confirm I have done all that you described and the problems have persisted.

Other advice I got was: you must not add decos while they are on an ethernet cable. That was crucial. 

So - what worked:

1. add Deco 1 - cable to the router. Set up network, switch to AP. All other decos are powered off.

2. add Deco 2 - via air - no cables! 

3. add Deco 3 - via air - no cables!

All works. 

Now another problem persists - if I now plug the last deco to the ethernet cable, after a while the network goes to red again. 

Network topology is as in the first description:

router - ethernet cable - deco 1

deco 1 - air - deco 2

router - gigabit ethernet - switch - gigabit ethernet - deco 3

I must unplug deco 3 from ethernet to make it work again, so the topology is :

router - ethernet cable - deco 1

deco 1 - air - deco 2

deco 2 - air - deco 3

No idea why.

  @RadekB 

It is possible there are issues with Ethernet wiring at your house, or gigabit switch is not compatible with Deco mesh. Really worst but very infrequent case is your ISP router not compatible with Deco mesh.

See more about it here: General questions about Ethernet Backhaul feature on your Deco

You can do simple test to confirm that. Using short Ethernet cables of good quality, daisy chain your three Deco units like this:

router - Ethernet cable - Main Deco (deco 1) - Ethernet cable - deco 2 - Ethernet cable - deco 3

If this Deco setup is stable and neither deco goes red, it means the issue is with networking gear or wiring at your house. To exclude ISP router as a suspect, when Deco is in Access Point mode (important!), and with short Ethernet cables of good quality, connect all Deco units directly to ISP router Ethernet ports:

router port 1 - Ethernet cable - deco 1

router port 2 - Ethernet cable - deco 2

router port 3 - Ethernet cable - deco 3

If Deco mesh has no issues in this setup, too, it means ISP router is no longer a suspect.