@Armin, so if I am understanding things correctly, you’re losing connection between your Win10 notebook (connected via WiFi to your WLAN Access Point) and your Roon Core, which is running on a NUC (wired connection to your router)?
I see that you are using a Fritz!Box router; which one? And what is the make and model of your WLAN Access Point?
The reason for asking is that my setup has a Fritz!Box 7360 and a Fritz!WLAN Repeater 1750e in it. I was being plagued by connection problems until I ensured that all the laptops that were acting as both Remote and Output devices were connected to the 1750e’s 5GHz band using the ac protocol, and I turned off the WLAN capability in the 7360.