I’ve had similar problems with getting into the device via the file browser when it works just fine through the website. These may seem like obvious questions, but are worth checking off:
When you log in using your admin credentials in the setup page, have you created yourself a username and password? This will be the account to access your stuff with. (I recommend not making your username an email, my problems seemed to come from the @email.com portion of it). My username is just my first and last name. Once you’ve got that set up you will need to map the server to your network drive. There are instructions on how to do so in the mycloud literature, i’m sorry I don’t have it in front of me otherwise i’d link you to it. If you look up “how to map MyCloud to network drive” you should find the results very quickly. When you go to map the drive it will ask you for your log in credentials, the ones you made for yourself on the mycloud website. Enter those and you should be good to go and you can access your server within your file browser.