@Rodarius 

When home network doesn't have underlying problems, none of these issues should occur with Deco mesh running in AP mode. 

Here is what I can recommend:

Connect all three Deco M4 to Ethernet ports of your router. Routers usually have four ports, use three for Deco units connected with short Ethernet cables, use fourth port for your home wired network link. 

Verify that with this setup your home network behaves. If it does, start taking Deco M4 one by one, starting with Main Deco, and bring them to their permanent place. Main Deco first, then verify if network still behaves. Then, one of satellites and verify again. Then, same with last satellite. 

This should help to pinpoint to what triggers the problem. 

  @Alexandre. 

Thanks for the input, It seems the issue is with the Smart DHCP feature, which cannot be deactivated and wrecks the network, when active as I already have a seperate DHCP on the network.

  @Rodarius 

So, the underlying issue on your home network is your DHCP server becomes unavailable (goes down). This is requirement to trigger Smart DHCP feature in Deco. So, Deco behaves as a canary in the mine, so to speak.

You'll need to figure why your DHCP server is not reliable. 

Also, as soon as your DHCP server comes back, Deco mesh should turn off its DHCP service. Because Deco DHCP service, when running, provides IP addresses with very short lease time, home network is supposed to recover in under 1-2 minutes, and that is only for devices that tried to renew IP addresses when your DHCP server was not available. 

I dont yet know, what triggers the Smart DHCP, as the Pi-Hole is a very reliable DHCP server, but in any case the Smart DHCP doesnt seem to switch off again.

Also, I have zero interest on having a feature on the network, that is beyond my control and can wreck havoc over the network, absolute showstopper for me.

I can easily work around an issue with DHCP outage, but having 2 on the network creates massive headaches.

Unless I can disable that behaviour completely, the devices are out of question to use.

  @Rodarius 

You will need to check setting on your DHCP server which is responsible for "do not start DHCP service if another DHCP server is already running on the network" and turn this setting off. 

If you can force your DHCP server to always start its service after shutdown , reboot or failure, your home network will have way less issues with Deco mesh, if at all. 

It is assuming your DHCP server is as reliable, as you think. 

  @Alexandre. 

GIven the thread I found, with 14 pages of people complaing about the Smart DHCP, I would more believe, that this is an issue,  that TP-Link should address and make optional. 

I havent had a single issue with my network for years and years - adding the M4 Mesh and its Smart DHCP has caused more headache and work, than any other network device before. I have no idea, why anyone thinks a random DHCP server beyond ones control is a good idea.

  @Rodarius 

Smart DHCP is unnecessary feature, no discussion about it. 

Still, it does expose underlying issues with home networks, and on this thread you found there are people that just upset they brought the canary in the mine and it died. It must be canary fault. 

Smart DHCP feature will not be triggered if Deco mesh does see DHCP server. If you don't want to figure why your DHCP server disappears, so be it. 

Your DHCP server must also have Smart DHCP feature enabled by default, and it collides with Deco Smart DHCP. If you don't want to find how to turn that feature off in your DHCP server so that it will always restart its dhcp service, don't place blame on Deco alone. 

If I sound harsh, it is because I expect a bit of IT competence from someone running their own DHCP server. If you don't want to use that competence to figure underlying issues with your home network dhcp service, getting rid of Deco M4 would be the right thing to do. 

"getting rid of Deco M4" .. given the quality of support i received and following this thread, that exactly what I am going to do. Thanks for confirming, that TP-Link is not for me

Also "Smart DHCP is unnecessary feature" yet I am to find that unnecessary feature on my setup to cope with TP-Links unnecessary feature?

Hardly makes sense, does it?

  @David-TP 

All is working fine until I turn on the M4 Mesh. Then all wired machines I switch on or reboot complain about duplicate network addresses. The instant I switch off the M4 Mesh or disconnect it from the switch, all returns back to normal.

My only explanation is some rogue DHCP.

[Edit]

I currently have reinstituted Asus AI Mesh in place and everything is working fine. The TUF-AX5400 boxes are setup on the exact same cables and spots as the M4

[Afterthought].

Cabled devices can find both the original DHCP and the M4 on the wired net, the WLAN devices would only be fed by the M4, if it decides to run its own DHCP