Here is a suggestion. First, map the My Cloud Share(s) that you want Plex to use as it’s media library to your computer.
Then add those shares to their respective Libraries in Plex by select Manage Library, then Edit, then Add Folders, then Browse for Media Folder. From there you should be able to search for and add the mapped My Cloud Shares.
Or as the prior poster indicated one could just use the My Cloud embedded DLNA media player and a DLNA client on their devices. Most smartTV’s and similar devices have either a DLNA client that is included or that can be added that will browse the local network for DLNA media servers and stream media content from them.
Edit to add: Also note that if the Plex server is installed on a computer that uses WiFi to connect to the My Cloud and to the device that will stream media you may run into buffering issues while streaming media. The cause is the streaming from the My Cloud then to the TV or other device potentially saturating the computer’s WiFi connection. If possible, it is best to use Gigabit Ethernet connections on all devices, particularly the My Cloud and the computer running the Plex server.