TL;DR: Every Fritz!Box has been compromised by DNS hijacking and using a Fritz!Box is unsafe. Every user of a Fritz!Box should immediately stop using their Fritz!Box or set up their own DHCP server.
Fritz!Box modems are popular modems by the German company AVM. They are tailored to the some-what more experienced users who want to have more control over their network settings and they can often be used to replace the modem from your cable/DSL/fiber ISP. The company has been around for a long tim
EDIT: Apparently the domain has been removed. Try yourself by visiting https://fritz.box/.
Just run Pi-hole as your DHCP and you’re good.