@Super-Ingo 

Hi, 

Please go to Deco APP>More>QOS and make sure it is set as 1000m/s download/upload.

Then check the mobile devices are connected to 5 Ghz, instead of 2.4 Ghz.

You could temporarily turn off 2.4 Ghz under Wi-Fi settings>Advanced.

Thank you very much and best regards.

  @Super-Ingo 

Hi, sorry for the delay.

I think you might set Deco X50 in Access Point mode, and AP mode doesn't support QOS.

In other words, it will no longer limit the wireless speed.

Is the 100m/s tested when mobile devices were connected to the first main Deco X50, or one of the satellite Deco X50?

Thank you very much and best regards.

  @Super-Ingo 

Hi, Thanks for the update.

For the network layout like:

Internet---Fritzbox---<cable>---first main Deco X50---<WiFi>---satellite Deco X50#1----<WiFi>---satellite Deco X50#2

The wireless speed from the satellite Deco units, especially satellite#2 would be less than the main Deco.

(Usually 30-50% loss according to the wireless distance and interference after one hop).

Why is my satellite Deco unit not getting the same speed as the main Deco?

The Ethernet Backhaul  is always a good option to improve the Mesh speed and stability:

General questions about Ethernet Backhaul feature on your Deco

Best regards.