First, if you have not already done so read the My Cloud User Manual. See Chapter 8 - Accessing Your My Cloud Device Remotely. Or see the Help section within the My Cloud Dashboard.
Second, for remote access to work properly one needs to enable Remote Access via the WD My Cloud Dashboard. See Dashboard > Settings > General > Cloud Access > Remote Access = On. Once remote access is enabled one then configures each User account on the My Cloud for remote access via the Dashboard > Cloud Access.
Once remote access has been enabled one can use the My Cloud Desktop (for PC or Mac), the My Cloud app (for Android or iOS), the WD Photo app (for Android or iOS) or the MyCloud.com web portal to access the content on one the My Cloud remotely.