Setting Internal DNS Server on RAX40

I have a pihole i want to use as my internal DNS server, but I see no where in the router settings to set what DNS server the DHCP gives out to internal devices.  I dont care what server the router uses externally as I wont be using the router for DNS 99% of the time but I need to be able to set what dns server is handed out to devices via DHCP from the router.  

I don't really want to resort to having DHCP run off some other device...  am I missing something?