Goto the WD My Cloud dashboard > Settings > General > Remote Access section. What is your connection status?
If you want to open the port, in the Remote Access section in Settings > General, click configure, manual, set both HTTP and HTTPS to a custom port other than the ones listed and save.
In your router, look under either the firewall or security advance settings and look for port forwarding. "IP" would be the IP of your WD My Cloud, "External port" would be the custom port that you set on the WD My Cloud, "Internal port" would be port 80.
Do another port forwarding, this time for HTTPS, Internal would be 443. See example circled in red: