In Tableau Mobile iOS the contents not showing

Dears,

Tableau Mobile in iOS is not showing the contents, when we open a report created, it is showing the error like

"Tableau cannot reach the server, Reload View"; the content is not loading. Sometimes, can open and see the contents.

But it is working smoothly in Android phone.

By the way, we use VPN to connect to Tableau Server.

Someone please help how to resolve this issue. 

Thanks.

Best Regards,

Chan