I use the same username to access the device remotely, and it’s not working on the app
Are you using the OS5 My Cloud app or the OS3 My Cloud app? If using OS5 one should download and install the OS5 My Cloud app to access that OS5 device.
One may want to see, if they haven’t done so already, if there are any relevant WD Knowledge Base Articles that may address their issue in the following link.
My Cloud OS 5: Online User Guide and Solutions