There is a class of internet users that doesn't use a search engine, and instead types things like "usedcarsdetroit.com" in the URL bar until they find something they want.
And there is the class that starts with a single word search like "cars", which can't go straight to Google. It instead has to see if "cars.fritz.box" exists, because perhaps there's a server called "cars" on your network.
And god forbid that your parent types "bankofmerica.com" instead of using a bookmark or their smart phone app.
Setup a local DHCP server, and don't set the fritz.box search domain.
Ensure that DNS-over-HTTP (DoH) is enabled where it can be.
Set upstream DNS servers that block malware, such as 1.1.1.2 or NextDNS
Delete "fritz.box" from the domain search list in DNS settings.
Educate your parents to be cautious about directly typing domain names or searching from the OmniBox.
https://nextdns.io/
https://blog.cloudflare.com/introducing-1-1-1-1-for-families...