Looks like your not able to assign LAN DNS servers via DHCP with this router.
Alternative is to disable the DHCP service on the router and instead activate the one from Pi-hole:
Ps. if you post router brand and model, someone else might confirm.
The router doesnt need to know.
The router just routes network packets from LAN to WAN (the internet) without caring who they are from.
After you switched DHCP server, make sure the clients renew their DHCP lease by either disconnecting & reconnecting network or reboot them.
Ps. I adjusted subject a bit for others to find if you dont mind.
I think you need to set you IP of the Pihole outside the DHCP Range. eg 10.0.0.100
Run pihole -r to Reconfigure and change the IP address.
Then go to the WAN IPv4 settings page 26 and select you Pihole IP as a static DNS IP address.
Page 26 refers to the WAN settings for the router.
You could configure the router to use Pi-hole as upstream DNS resolver.
But this will break stats being displayed on the Pi-hole web dashboard as only the router requests will be displayed and you wont see proper stats for the clients.