When you connected it to Ethernet and your main router did the LED act any differently?

What is a Sonos AMP gen 3? Can you post a picture of the label of this device, obscuring any PID?

No, the LED did not change when I connected it via ethernet.

Time to call Sonos Support.

Hava Getlam ürün bu AMP GEN3

Mod edit: Removed serial number and pin

ethernet kablo ile bağlantılı video da çektim fakat yükleyemedim  

​@Ozgur That’s a Sonos Amp (that is only made in one version so no generation is mentioned). The predecessor to the Amp was the Connect Amp, that was made in two generations.

I (Google-) translated your last post:

ethernet kablo ile bağlantılı video da çektim fakat yükleyemedim 

I also shot a video with an Ethernet cable, but I couldn't upload it.

You could upload it to YouTube and share the link here.

Yes, I finally contacted Sonos support, but they couldn't do anything, they decided that the device was faulty (I expected them to say something different), and we couldn't get any results because it was outside the warranty period.

You or anyone else looking here should be informed, if you encounter such a problem, do not bother and throw the device in the trash.