Green internet light goes blinking and no-blinking time to time

Since yesterday evening, I have very unstable internet, which align with the behavour of the internet light on my KPN box 12’s internet light. It keeps on switching between no-blinking to blinking in intervels of a 30s to 2min.

I have tried the following but nothing works: reconnect the fiber connection box, reboot the box, reboot router, reset router, replace cable between the box and the router.