The My Cloud User Manual explains in general and simple terms how to access, both locally and remotely, the My Cloud device and how to configure the various features.
https://support-en.wd.com/app/products/product-detail/p/126
One uses the My Cloud Dashboard to configure the My Cloud including setting up or changing the Remote Access/Cloud Access feature.
How to access the Dashboard on a My Cloud (single bay) device
If you have forgotten the administrator password for the My Cloud (may be needed when accessing the My Cloud Dashboard), one can perform a 4 second reset using the reset button on the back of the My Cloud device to reset the administrator password back to default (blank).
How to Reset a My Cloud (single bay) Device
In certain cases one’s broadband/ISP provider may be blocking or restricting access to the default ports (or UPnP) used by the My Cloud for remote access/cloud access. If this is the case, one can change the My Cloud Remote Access from Auto to Manual in the My Cloud Dashboard > Settings and use alternate ports (ex: 8080 and 4443). When using the Manual option for Remote Access one typically also has to access their network router and configure port forwarding for the two ports to point to the My Cloud device.
Another WD Support Knowledgebase article that may be relevent when one is having remote access troubles.
Remote Access - MyCloud.com Device Offline or Cannot Connect to My Cloud
Error
For local network file access to the My Cloud, one would use Windows File Explorer or Mac Finder.
For remote access to the My Cloud, assuming remote access is enabled and users granted remote access, one would use the MyCloud.com web portal or the My Cloud app for iOS or Android. WD removed the capability years ago to remote map a My Cloud using Windows File Explorer or Mac Finder.
In many cases simply turning off Remote Access/Cloud Access in the My Cloud Dashboard > Settings. Waiting five to ten minutes. Then re-enabling Remote Access/Cloud Access will reestablish the secure tunnel used for remote access. Other times if the problem is on WD’s end one has to wait for WD to fix the issue on their side before remote access resumes working.
It should be noted that using a Internet of Things smart plug to turn on and off the My Cloud may cause potential issues of the My Cloud not shutting down properly or even the possibility of data corruption. Generally NAS devices like the My Cloud are designed to be left on 24/7. Turning them off without properly shutting them down may cause possible issues if the My Cloud is writing data to the hard drive when power is removed.
Generally the My Cloud devices (and other NAS devices) are not simple plug and play devices with respect to remote/cloud access. There typically is some configuration that has to be done within the My Cloud Dashboard to get the remote access feature to work properly and for users to have proper remote access.