Pihole should be fairly easy to use. Either run it on the server and use the servers ip address for the dns server for the clients.If this doenst work (you have another dns service on the unraid server) use the set separate IP address for a docker instance detailed a couple of posts back.
If you want to disable pihole on one of all clients, just change their settings to use dhcp to get their settings from the router or change their dns settings to google 8.8.8.8 and 8.8.4.4 and reboot clients.
Pihole doesnt affect plex directly - it may just be your dns settings on the clients