When on cellular data (4g), how are you attempting to access the My Cloud? Are you using the WD mobile app for Android/iOS (http://www.mycloud.com/#/apps_os3)? Have you enabled remote access/cloud access through the My Cloud Dashboard and grant one or more My Cloud Dashboard User accounts remote access?
If you have not already done so you should read through the My Cloud User Manual which has specific chapters covering how to enable remote access/cloud access to the My Cloud device.