Brandon Sanderson is awesome. I recommend starting with the mystborn books, but stormlight are good as well. They're both part of the cosmere, a universe created by Sanderson. Many other books as well.
Ok, I see the problem. Your router needs an external DNS server for it's internet setup.
You need to set DHCP to give your pihole server as the DNS to the computers INSIDE your network. It's impossible for your router to use your LAN DNS server on the WAN port.
I've never heard of spectrum doing this. I think it's an issue with your router. The steps you mentioned sound right, but I'm not seeing what you're seeing. I usually try to look at the advanced mode, which might have more info.
You could contact Netgear tech support, or consult their manual. Have you made sure you're on the latest firmware?
Historically I've used debian based distros on my home server. I recently switched to opensuse tumbleweed and Ive been very happy. It's a rolling release, and I've found it more stable and easier to work with than linuxmint