I do run Pi-Hole and Ad-Guard on two different Pi's. Yes you can configure two different DNS (IPv4 and IPv6 if you provider supports it) under the tab provider access credentials or something like it, I don't know what the english tab is named. Also in the DHCP settings is a tab to configure two DNS, also IPv4 and IPv6, to be rolled out to the clients.
As macom wrote, provider fritz!boxes have different firmware and often not all features are enabled. Better buy a retail box.