@Alexandre.

Again thank you for the clarification. In the meantime I have reconfigured my network. Now it looks like as follows:

ZTE MC889 router (ISP)  <--- (cable) --->  TP-Link WR841N  <--- (cable) --->  Raspberry Pi

                                                                                                  <--- (cable) --->  Deco 1  <--- (WiFi) ---> Deco 2

                                                                                                                                          <--- (WiFi) ---> clients (notebook, Sonos, ...)

Notes:

  - The TP-Link WR841N has WiFi disabled and does DHCP and port forwarding

  - The Decos are in access point mode, standard settings are applied in the app (and they were correctly removed from the network before)

Unfortunately I still can only access the services on the Raspberry Pi when I am connected to the Deco 1. :-(

But right now I made an interesting observation: on Deco 2 I can access (some of) the services using the IP address of the respective service, but only partly. For example PiHole runs on 192.168.1.22 and when I enter this IP address, I get a "403 Forbidden", which is okay. When I call "192.168.1.22/admin" (which is the admin panel), the page loads until it generates an error, but the page title is correctly displayed in the browser tab! This also applies for other services that are running on the Raspberry (zigbee2mqtt, ...). So there seems to be some sort of connection, but the connection times out after some time. In Firefox I tried the developer panel and I see indeed some of the data from the pages are correctly transmitted, but then I receive an NS_ERROR_NET_RESET. As I just discovered, I also can ping the IP addresses in question. What the hell am I overlooking???? Question to myself, not to you! ;-)

Ah, and as always: browser cached deleted, network restarted, different browsers, ...

Greetings,

an even more baffled Christian

  @cgaertner 

The final test would be to enable WiFi on  TP-Link WR841N, turn off Deco mesh and see if connected to TP-Link WR841N by its WiFi everything works as you expect.

If it does, you'll need to give up on Deco. Disappointing, and frankly I was very confident in that setup you've made everything should work, but if it does not - you can't do anything about it with Deco mesh.

  @Alexandre. By the way, how do you get to update Deco page?

Tnx

  @Alexandre.

I did the final test yesterday and disabled the Decos and only used my old WiFi router. Unfortunately everything worked fine and so I came to the conclusion that my setup is not compatible with the Deco system. :-(

This morning I felt somehow angry that I cannot use the Deco system although it performed very well (at least the devices connected to the first Deco). The network throughput was significantly better than with my old WiFi router. Additionally your statement about the non-standard use of multiple IP addresses on one device came to my mind and so I decided to tidy up my Docker system. Thanks to the docker-compose files changing of IP addresses and ports is not so much work at all. Finally it took still some hours to find all dependencies between containers and in the configuration files of the running software. But now I have a clean system running on the Raspberry Pi, with only one network interface in use and only one IP address.

And as a last step (because now I wanted to know!) I reset both Decos, put them to router mode and set up the mesh network again. And - hooray! - it works! I tested everything for about one hour with different devices connected to Deco 1 and to Deco 2 and everything seems to work fine.

So what remains to do? Of course saying a huge "Thank you!" for your support, for your input and for your recommendations!! :-)

Greetings,

Christian