Unable to use My Cloud from external network

I am having an issue with My Cloud.
Sometimes happens that I can’t reach my NAS from external network using My Cloud.
There are no issues if I try to use My Cloud while connected on the same network of the NAS.

It is pretty strange because sometimes I can reach the NAS and sometimes not.
In the Cloud Access Settings it says that the NAS is connected and port forwarding established.

NAS Firmware 2.31.149

Thanks in advance

The My Cloud has an incorrect port forwarding address due to router reboot. Even if you did not cause the reboot, other reasons can cause a reboot.

Solution is to refresh the forwarding address by 1) turn off Cloud Access for a few secs and back on, or 2) just reboot the NAS.