The Avast is running in the background and then it reported this message (within its program window) regarding the Internetbox as follows:
"DNS queries on your device have been hijacked. You might be redirected to fake sites where your personal data could be stolen. Check the DNS addresses on your router and in your computer's network settings. If the network you are currently using is not your home network, use Avast SecureLine VPN to establich a secure connection. (Then comes a clickable link to a web site.)"
Swisscoms advice was to "clear the DNS cache, this means: results of a DNS query (when you visit a website for example) are stored to your Internet-Box cache. This reduces the website loading time the next time you visit the website. The DNS cache is activated by default. If you have further questions you can ask Avast or we have a special team for those special asks called my service". A service, first free, after six months chargeable, Program Internet Security, could be used to prevent further interruptions.
Maybe I just should empty the DNS cache in the Internet box. Otherwise I have both Avast and Malware running in the background.
You can be sure that your Internet-Box is not hijacked. Unfortunately AV - programs sometimes do not work properly. By the way: Avast is a really awful program that is selling you data. If you use windows 10 just use Windows defender. That is more than enough.