Hello jnbec4ever,
I had the same problema and solved a few weeks ago.
The way to do it is:
In Airport Utility you have first to Make a IP reservation:
Edit>Advance Settings > DHCP and NAT >Reservations > New Reservation > By MAC address
Input WD Cloud MAC address > Select a IPv4 IP address for the WD Cloud
Go back > Port Adjustments > New Entry > Choose IPv4 > Select a Service > Choose Access to a remote folder
Description input whatever you like,
Port UDP Public input 80
Port TCP Public input 80
IP address, input the one used in Reservations
Port UDP Private input 80
Port TCP Private input 80
Go back and Select New Entry again and do the same steps using 443 in all the ports as you did in the previous steps.
You will end up with a New Reservation and two different entries in Port Adjustements
OK everything and let the Airport Extreme reboot..
Then Go to WD MyCloud Control Panel > Settings > Network > Network Services, choose Static and input the IP you selected
in the Aiport Extreme.
Then Go to Settings > General > Cloud Access > Configure, select Manual, for External Port 1 use 80 and for External Port 2 use 443, then sabe..
After you saved all the settings, UNPLUG THE WDMYCLOUD FROM THE POWER LINE, wait a couple of minutes and then plug it again, it will reboot and it will be working fine and if you go to WDMyCloud > settings > General > Cloud Access, you will see Port forwarding connection established.
It is very important that you unplug and reboot the Cloud, this way it will use the settings you selected.
I hope this help you, let me know.