In DD-WRT settings: Services -> Services -> Dnsmasq and in the additional options write the following:
dhcp-option=br0,6,192.168.x.x
br0 is the interface you want to configre it for, 6 means you’re configuring the DNS server and it’s followed by the IP address of the DNS server(s). You can use multiple servers just by adding another address separated by a comma. You can find all the DHCP options from IANA’s website and in RFC 2132.
For example, I have the Pi-Hole configured as the first server, CloudFlare as a secondary and also CloudFlare is set as the server for the guest WiFi VLAN.
dhcp-option=br0,6,192.168.1.3
dhcp-option=wl1.1,6,1.1.1.1,1.0.0.1,8.8.8.8
Remember to replace the IP address with the address of the device where Pi-Hole is running.
If you want Pi-Hole to show the names of the devices and not only IP addresses, you can go to Settings -> DNS -> Advanced DNS settings -> Conditional formatting and enable conditional formatting, set the IP of the router and domain name to the one you have set (by default it is local).