Adding to this, your router is probably functioning as the DHCP server for your network handing out network details to the clients such as IP address, gateway, time service and DNS servers.
So you need to find the DHCP server section on the router and see if you can change the DNS server setting handed out to the clients into that of Pi-Hole.
If not possible, do as @DL6ER suggested and flip on DHCP on Pi-Hole ... after turning DHCP server off on the router.