If there is no way to push only a single DNS server (the Pi-hole address) via the routers DHCP service to the clients, you could consider letting Pi-hole do DHCP for your network instead of the router:
If there is no way to push only a single DNS server (the Pi-hole address) via the routers DHCP service to the clients, you could consider letting Pi-hole do DHCP for your network instead of the router: