Sir, I also wish to use a Raspberry Pi as a Pi-Hole DNS Server. In my TP-Link VR1600v I can put in two IP addresses for DNS Servers. I plan to point the first entry to my Pi-Hole and the second entry to CloudFlare or Google. I would do this to have a Public DNS Server as a "backup" in the event the Raspberry Pi fails. As long as the Pi-Hole on the Raspberry Pi is available will the modem ALWAYS use it or does it "Load balance" between the Pi-Hole and the Public DNS Server? If this is the case the Pi-Hole would not be the only DNS Server for my network? Thank-you.

@DownUnder  Usually every devices who use DNS and allow to use more then 1, use them as backup.

Mean that if your DNS server available, then it will be used, and if not, the backup DNS will replace the main.

I'm not sure if you need to reboot the device to use again the main DNS, or just wait until the DNS will be available again,

It's depend on the software used in your Router / PC.