Guide (recommended for experienced users)
Setup Pihole like described on their website
also setup the DHCP for the PIhole (IPv4 & IPv6)
then connect the pihole to the modem
deactivate the DHCP of the modem (IPv4 –> Deactivate / IPv6 –> Stateless)
after that you need to make 2 IP filters on your modem to block all outgoing requests from the modem itself.
restart the modem
then the pihole should be the only reachable DNS server in our local network. All request that go to the UPC DNS get automatically blocked by the modem