I was setting the Customer Upstream server as per the instructions, Method 2. Don't know if these instructions are outdated with the latest updates.
Anyways I removed the Custom Upstream server and selected to use the Cloudfare servers. I also removed the secondary DNS servers in my Router > Basic > Network > "Static DNS" fields and my clients still aren't connecting to the Pihole after a ipconfig /renew + ipconfig /flushdns + router reboot + RPi reboot.
However and after making these changes, I cannot ping websites on both the RPi and the client machines. I can still ping using the IP addresses.
I also tried setting up according to these instructions, however when I run dig pi-hole.net @127.0.0.1 -p 5353 , I get the following:
pi@raspberrypi:~ $ dig pi-hole.net @127.0.0.1 -p 5353
; <<>> DiG 9.10.3-P4-Raspbian <<>> pi-hole.net @127.0.0.1 -p 5353
;; global options: +cmd
;; connection timed out; no servers could be reached