This has been tested as working on Fedora 32.
First add the below to the [main] section in /etc/NetworkManager/NetworkManager.conf
dns=none
This stops Network Manager from overriding /etc/resolv.conf.
You can then edit /etc/resolv.conf and add a line for your DNS server, as below.
# Use Pi-hole for DNS:
10.0.0.3
You can then check it works by issuing a dig <address>.com and you’ll see the active DNS server in the SERVER section.
; <<>> DiG 9.11.22-RedHat-9.11.22-1.fc32 <<>> google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 7708
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;google.com. IN A
;; ANSWER SECTION:
google.com. 2 IN A 0.0.0.0
;; Query time: 21 msec
;; SERVER: 10.0.0.3#53(10.0.0.3)
;; WHEN: Mon Aug 31 21:42:12 BST 2020
;; MSG SIZE rcvd: 44
You may need to also run the below.
sudo systemctl restart NetworkManager