As a troubleshooting step, use custom port 32400 not 29999 (adjust the port mapping on the router to match) and see if the issue persists. You may have to reboot the router also. If it works then the problem is using port 29999 which may be blocked due to possibly being a port used by malware (see this link for that port).
Otherwise, since this appears to be a Plex specific issue not a My Cloud issue, you should if you haven’t already, see if there is a solution over on Plex Support or the Plex forum(s).
