Hi,
Unfortunately, Power BI Report Server does not support the latest Custom Visuals API, which we have to use to build the visuals. The newest version does not require internet connectivity at all. We are working with Microsoft to get this solved and we will update you when we have more details (@Aivis, please make a note of this).
We can provide you a version of the visuals, that is older, which works in the Report Server and does require an internet connectivity - would you like to use that version?
Thanks,
Support Team