One of the reason why Jet Hub is introduced is to reduce the need to install Jet Excel add-in for Viewer and currently Jet Hub does not support NAV User Password authentication if the viewer didn't key in their NAV credentials via Data Source Settings.

Jet Hub should have an input box to allow Viewer to key in their NAV credentials and update the password, the easiest way is probably allow the Viewer to see some of the data source settings in Jet Hub if it is running NAV User Password authentication.