My pihole serves dns. If not found, it goes directly to root tables (I forgot how they are called).
The router, I just connected its WAN port to the ISP's switch/router/AP. Within the LAN under my router I have DHCP sending everybody to do lookups to the pihole. I don't know what full bridge is.
The ISP's modem/router/switch/AP, I cannot configure. It is a fucking "smart" brick remotely controlled.