When setting up new nameserver's, you may see errors occur during initial testing, such as "One or more of your nameservers did not respond."
This error can happen due to several reasons. However, the most common cause is usually related to port 53 being blocked or filtered for incoming DNS requests.
You can manually verify whether the ports are open from the outside, by testing them from a remote server with the telnet command:
telnet $SERVERIP 53
You can also query the server directly to see if it responds with records:
dig @ServerIP A +trace yourdomain.tld
Depending on your hosting provider, the steps necessary for opening the ports may vary. Our cPanel documentation here covers our requirements on how to configure your firewall for all cPanel related services: