@JowivK 

Do you still observe that the ISP modem does not convey the trafic to the desktop computer conected to the modem directly?

If you run the Deco in Router mode, it is not allowed to connect a satellite Deco to the ISP modem.  That could perhaps explain some of the anomalies you observed.

When running in Access Point mode, it is possible, when the Deco detects that the modem is not active, that the Deco starts distributing IP addresses in the .68. range.

What I would do in your case:

1. disable the wifi on the modem and power off the satellite Deco
2. verify that a computer connected to the modem works well
3. setup one Deco in Router mode, connected to the modem.  This is probably not optimal, but it should work for the majority of the usage.  It should not cause interrupts and slowness.
4. change the Deco to Access Point mode, and verify everything works well
5. add a satellite Deco to the Deco mesh (the satellite should be blinking blue before you start that, and I suggest locating the satellite close to the main Deco until it is integrated in the mesh)

  @yves_b 

Hi and thanks for response.

Yes, the desktop computer works perfectly with the ISP Router-modem xombination device. It is connected by ethernet. Works wrll when both Decos are powered down.

And for reasons of my location, I cannot replace the ISP router combination with a Deco.

 The Deco must go to another floor, connected qith a good ethernet cable.

This Deco needs to be in AP mode. Although  I can set it up in router mode (jtbis working like that now, but the WuFi devices connected jere are loosing connection often.

The second Deco goes to another part of the house again, but I can put it next to the first Deco for setup.

But I need to run both Decos in AP mode in the end.

  @4yves_b 

Hi again. 

As responded above, I don't have the option to run one Deco as router. They must both be in AP mode.

Unless: Could I run two DHCP servers? The main router could be set  so the range above 192.168.1 100, and the Deco (if it only works in Router-mode), could manage the range up to .99? But those would obviously be in that 192.168.68.xx range anyway. Or could I set it up in the 192.168.1.xxx range? But for some reason, that change seems impossible for that Lan IP? Is there no way to disable that LanIP setting in the Deco? What good is it?

I have thought about assigning all the Wifi devices fixed IPs instead of waitingbfor the DHCP. But, as the Deco is in router mode now, I can only assign in the 192.157.68.xx range? Or can I try to go around this Deco DHC s server and assign those on the ISP router?

Or should I deactivate the DHCP server in the ISO router and only use the Deco server?

All this is pretty mind boggling for me. I'm a 76 year old lawyer, not a networking expert

  @JowivK If you make a single mesh with both Deco, you do not configure the individual Deco.  You configure the operation mode of the mesh, you do not configure the operation mode of the individual Deco.

Did you attempt following the 5 steps of my suggestion?

If you decide configuring the Deco mesh in Router mode:

1. the satellite Deco cannot be connected to the ISP modem
2. both the ISP modem and the Deco mesh will run as DHCP server, but for different clients.  The ISP modem will assign IP addresses to its clients, and the Deco mesh will assign IP addresses to its clients.
3. The LAN IP address of the Deco mesh must be outside of the subnet of the ISP modem.  Hence if the ISP modem assigns addresses in the 192.168.1.nnn subnet, it is fine for the Deco to have a LAN subnet with addresses in the 192.168.68.nnn subnet

  @yves_b 

Hi again and thanks for your feedback.
No, I have not reset my system yet accoring to your 5 steps, because you advised to connect the Deco to the modem. And I only have the TP-Link Archer device router inlc modem, I cannot connet the Deco only to the modem. I can only connect it to the router.

So, again: The main Deco will be connected to the router/modem Archer. via ethernet cable. That must be configured in AP mode eventually, even if I set it up as router first (it seems that there is no option to set it up as AP right away? I run the setup routine, and then switch to AP. Right?

I have done this about five times, the effect is always the same: As long as the Deco is in Router/modem mode, the Wifi devices work (most of the time, losing connetivity at irregular intervals). When I switch to AP mode, I can see the  IPs in the DHCP server of the Archer, and the Deco connetcts. But the devices have no connectivity. Thats why I thought I should give them static IP addresses, but that doesn't seem possible. 

Before I have this main Deco working stable, I will not try to conect the second Deco in the mesh. I understand now, that this mesh will be a a second IP range in the 192.168.68.xxx system, So it will not be possible to give the devices static IPs in the Archer DHCP network?

I am grateful for your explananation about the different DHCP subnets. Now I understand that a bit better.  So when I do establish a Deco as AP, this gets its own IP in the 192.168.1.xxx range from the Archer, and uses that connection to set up its own DHCP server for the mesh, which will then be in the 192,168.68.xxx range. Correct? And if I should change the Deco's LAN IP from 192.168.68.1  to something else, that would only mean that all its devices will have their IPs in that subnet.

The thing about potential conflicts leads me to the following question: I can restrict the DHCP subnet of the Archer to exclude, say, anything below 192.168.1.50 and then assign the Deco an Lan IP in that "excluded" range, like 192.168.40.1. Or is that not usefull or cpuld cause other problems? Or is it impossible that there is interference between the two DHCPs?

Thanks for your patience!

  @JowivK 

I understand you have a single ISP box, "TP-Link Router with integrated modem that has been provided by my ISP", hence I called it an "ISP modem".

Indeed, the main Deco will be connected to the router/modem Archer. via ethernet cable. That must be configured in AP mode eventually, even if you set it up as router first (it seems that there is no option to set it up as AP right away.  You run the setup routine, and then switch to AP. Right!

Did you disable the wifi of the "ISP box"?

If you run the Deco in Router mode, then your "ISP box" sees only the Deco as client, sees only the IP address of the Deco (assigned by DHCP by the ISP box), and the Deco will assign, via DHCP, "private" IP addresses to the clients of the Deco.  In Router mode, the Deco hides its clients to the ISP box.

When you establish a Deco as Router, this gets its own IP in the 192.168.1.xxx range from the Archer, and uses that connection to set up its own DHCP server for the mesh, which will then be in the 192,168.68.xxx range. And if you should change the Deco's LAN IP from 192.168.68.1  to something else, that would only mean that all its devices will have their IPs in that subnet.

In Router mode, Deco should not accept configuring its Lan IP to an address in the 192.168.1.nnn range.  It is not allowed to have an overlap between the WAN (Internet side) range of the Deco (192.168.1.nnn, announced by the Archer) and the LAN (local side) range of the Deco. 

When you run the Deco in AP mode, the ISP box will assign IP addresses, via DHCP, to everybody.  The devices connected to the Deco will obtains addresses via the DHCP server of the ISP box.  The ISP box will see all clients as if connected via the Ethernet cable from the box to the main Deco.

When you establish a Deco as AP, this gets its own IP in the 192.168.1.xxx range from the Archer, all devices connected to the Deco will also obtain, from the Archer, own IP in the 192.168.1.xxx range.  The Deco does not run as a DHCP server, and you should not have the option to change the Deco's LAN IP.  Actually, the Deco (in AP mode) does not manage IP addresses for its LAN.

When the Deco is in AP mode, you wrote that "the devices have no connectivity".  Are those devices connected to the wifi of the Deco?  Do those devices receive an IP address?  Which one?  Does the Deco light remain solid white?

  @yves_b 

Hi again and thanks for these clarifications. I think  thanjs to that, I am beginning to understand how this thing works.

Today, we have an electricity outage, so I can't do anything. But tomorrow I will try the big reset.

Jopefully it will wirk. Why do I have to switch.off the Wifi of the Arvher ISP?.I would like to have that as an option in that room. Of vourse not as part of the mesh, and with a different SSID.

But for setup it I can switch it off....

  @JowivK You are right, you can keep the wifi of the Archer, just keep it seperate (other SSID) than the wifi of the Deco.

  @yves_b 👍😘👍

  @yves_b 

Hi again and time fpr final feedback.

In effect: Problems seem solved

But one mystery remains.

I did eventually reset the main Deco, connect it as Router and switched to AP mode. And it worked. The main issue I had was that I could not set up the same Wifi network as before. Although I had done my best to delete that network SSID, and the Deco had been reset multiple times, it came poping up again and preventing me from using that SSID again. So I had to set up with a new SSID. (Yes, all different from the Archer router's Wifi SSID.) I looked for the device that was broadcasting that former SSID (with password check!), but I couldn't find it.

Yes, I have about 10 devices on the Wifi there, but none ever had modem connection or was set up as Router, None had a DHCP running. But there it was, and when I wanted to set the Deco Wifi up with that old SSID, it wouldn't do that. So I had to take a new SSID (which means reconfiguring the 10 WiFi devices, range extenders, smartphone, TV box, Audiocast device etc. But once done, they all worked stable.

And when I really placed the second Deco next to the first one, I could also set that up as satellite Deco, and it is working fine.

To find out where that mysterious Wifi comes from (still shown in the wifi options of all devices) I also checked the Archer router. It is an Archer AX 12. And when I looked into the settings, I found that Easymesh was switched on. I also remembered that in one or two instances  during setups I got the option One-Mesh device, but it disappeared as quickly as it sowed up. I don't remember ever setting up (or trying to set up a One.Mesh network or use EasyMesh in the Archer. (The Decos wouldn't support it anyway.)  But could it be that it got set up with that old Wifi SSID and is still ghosting around somehow?

Well, it's a minor concern. I just have to make sure that nobody tries to log into that WiFi network. As it has no WAN connectivity, it will be a short-lived try..

It's just kind of weird that a WifI network is in some way active in my house, with my password, and I cannot deactivate, delete or modify it because I dont't find the device which provides it. Could it be the Google Home app? I just have no idea!

Anyway, thanks for your patioent support, I did learn quite a lot over the last days. And seem to have a stable home network now.