Thanks @Alexandre, you pointed me in the right direction. Cable issue for sure, but finding WHICH cable will take some work. My setup is more complex: Modem -> Wall Box -> [Attic Cable] -> Wall Box...

I have two Deco M3. One is connected to my router using a cable. The other is connected to the first using WiFi. Both are in AP mode. I have to restart every day to get my 500 Mbit. After a while it...