Network error 500

I have just purchased a My cloud to store pdf files. It is connected to a Wi-Fi router. I need to access the files from 4 iPads that are linked wirelessly to the network. For security reasons the wireless router is not connected to the internet as we do not require internet access on this network.
From the IPad I can access the dashboard no problems, but in the mycloud app I can see the WDmycloud under local devices, but when I try to connect I get and error screen.

Connect the router to internet and it works fine, but for security reasons I don’t want to leave it connected.

Any advise would be much appreciated.
Kind regards

Considering E-mail credentials are at play I’d presume the unit would require Internet access even on a local connection when linking to the app.