@Gregor50 

Deco M5 flashing red is the problem that must be taken care first. In normal operating mode Deco M5 LED should be solid green. What if you disconnect Satellite Deco from the Ethernet cable, only leaving Main Deco wired, will they stay solid green? 

Also, see this document: General questions about Ethernet Backhaul feature on your Deco

@Alexandre. Yes, if disconnected from the Ethernet they are solid green. However the speed is not nearly as good as if they are connected to the Ethernet. 

@Gregor50 

I would recommend you read document that I provided. It explains how to properly wire Deco units.

Short document summary: if you run Deco in Access Point mode, you could wire Deco units any way you pleased. When Deco runs in Router mode, which is the mode it is set when initially configured, there are restrictions on how Deco units can be wired.

If you are after WiFi performance, I would recommend switching Deco to Access Point mode, this is how you can do that in Deco app:

As for Deco WiFi speeds, it would help if you tell what you are getting. For me, the best I ever got from wired M5 in speed tests is 475Mbps, average speeds are 300-400Mbps, with occasional dips down to 200Mbps. 

For wireless M5, the top speed in test I ever saw was 225Mbps. Average speeds are less.

All speed tests done on Android smartphone, with Ookla speedtest app, WiFi 2.4GHz band turned off and only 5GHz band is available, smartphone is placed 4-6 feet from Deco M5, in direct line of sight.