I suggest disabling DHCP on the Sunrise box and enabling it on the Pi-hole. Make sure that the DNS Option in the DHCP Settings of the Pi-hole points to the internal Pi-hole IP Address.
Then reboot the clients and you should get an IP address with the pi-hole as DNS server.
Just make sure that the configured pi-hole gateway address is the internal IP of the sunrise box.
You can’t define the Pi-hole in the DHCP options on the Sunrise box probably, so you need to disable that DHCP server.