@schillkroete 

Consider upgrading M5 to latest hardware firmware, as a first step. You may need to do it manually, by downloading it from TP-Link Web site of your region.

It might be possible that this old firmware (from 2017, really?) does not support modern security protocols for devices to connect to it.

@Alexandre. Thanks for your answer. I am trying to upgrade to a new firmware since hours. Does not work at all. Nor via web interface neither via M5 upgrade tool. Upgrade via App is not possible.

What do you mean with upgrading to newest hardware version?

@schillkroete 

In regard to hardware I made a typo, sorry. Have my mind preoccupied with hardware issues I am dealing with on my server, these are unrelated to Deco. 

With such an old firmware, upgrade through app won't work. If you can't upgrade M5 to latest firmware manually, try the previous, older version, available from TP-Link Web site. If that does not work either, try next one and go down the list till you find one that can be installed manually. After that, check in app if newer version became available and upgrade, if not, do it manually going up the list. At some point, in app upgrade will become available.

If nothing works and you can't upgrade firmware at all, escalate to TP-Link support and if they are of no help, return that M5 to seller.

@TP-Link 

Hi, thanks for your answer. I buyed it in a Shop for refurbished Hardware in eBay with 12 months warranty.

The LAN Signal is working. I tried that already. I can Test the wireless Signal this afternoon. The M5 does not have to be the Main deco, does it?

@TP-Link 

I tried that now. When I turn off all the P7 (one of the P7 is the main Deco), I can find a Wifi named "TP-Link". So I think the wireless function of the M5 does work.

When I turn on the P7 main deco again my devices somehow do not connect to the M5. I tried in my garage. I put the M5 into the garage an had poor Wifi connection even when I was standing directly next to the M5. When I had one of the P7 installed in the garage I had full Wifi all over the place. Same in my bathroom and other rooms.

Woud you recommend me to return the M5 to the seller?

@schillkroete 

If Deco M5 is connected to Ethernet, please disconnect it from Ethernet cable so that it will use WiFi connection to your Deco mesh. Then, please clarify the following for me.

1. LED on M5 is green.

2. You can see M5 succesfully added to your Deco mesh, in the list of Deco units in Deco app. Example:

3. With M5 is connected wirelessly to the rest of Deco mesh, you do see both 2.4GHz and 5GHz bands in M5 signal source. Example:

4. What is SSID name of your Deco mesh Main Network?

5. Are you OK to install WiFi analyzer app on your smartphone (I'll provide you with app name from Google or Apple app store) to check additional information that smartphone sees from that M5?

@Alexandre. 

Thanks for your answer.

I can See M5 in the overview. SSID is "Pretty fly for a Wifi"

But I do not See where to find the two GHz Bands. I activated both 2.4 and 5 GHz.

The offline Decos in the picture ate the P7 which I die not reconnect Yet. Dr is the M5.

I am fine with installing a Analyzer app.

@schillkroete 

In Deco app, on that screen that you copied to reply, touch Deco icon named Dr and new screen will open with Signal Source information in it. Please copy it to your reply. Example:

WiFi analyzer app name: WiFiMan by Ubiquiti. Link to Google app store: https://play.google.com/store/apps/details?id=com.ubnt.usurvey

Same app is available in Apple app store.

When you have it installed on smartphone, let me know and I'll tell you what to check in it.

@Alexandre. 

I took a Screenshot from both main Deco and M5.

M5 only Shows a MAC Adresse but no IP.

Analyzer App is installed