The website example.com is resolved from the Plesk server:
[root@pleskserver]# dig +short @pleskserver_IP example.com
The same website cannot be resolved from external locations:
[root@remoteserver]# dig +short @pleskserver_IP example.com
;; connection timed out; no servers could be reached
Firewall is turned off on the server.
Solution from this KB article did not help.
The port 53 is filtered. This can be check with the 'nmap' utility:
[root@remoteserver]# nmap -p53 example.com
Starting Nmap 6.47 ( http://nmap.org ) at 2017-12-05 16:29 +07
Nmap scan report for example.com (18.104.22.168)
Host is up (0.21s latency).
PORT STATE SERVICE
53/tcp filtered domain
Connections to the port 53 are filtered by an intermediate firewall.
Contact network administrator or Internet Service Provider for assistance.