router – Internet connection partly not working: Some websites do not load

I am on an internet over cable connection. Cable modem is a combined device including router and wlan hotspot.

Since yesterday I experience strange network behaviour – nothing changed on any settings or devices at all:

  • internet connection is generally available
  • however, some domains / websites are not available. When called in any browser on any device, browsers return timouts
    • note: those websites are working. I check this with my mobile connection (different provider)
  • I am on a DELL Precision laptop on Ubuntu 20.04. However, same behaviour can be seen on a windows PC and on an Android smartphone independently from any browser version at all
  • When I ping any such URL in a console it says 0 received, 100% package loss
  • There seems to be no logics in what domains or websites work and which not

I do not even have a clue, on what to look or to check? Any help is much appreciated. What further information would you need to narrow the problem down?