I have a Windows UWP app which we distribute using the public Microsoft store. Recently, we have a number of clients where the company has very strict internet access, and no access to this public store.
When I build the app, we get a "test" version where we can have it "sideloaded" via running a Powershell script. However this is not ideal as the user needs to enable unknown sources, and worse we also seem to need to enable developer mode, to accept the certificate. Also they lose the auto updates.
I have seen this post, but it is now quite old, and I could not get any real information from it.
For your scenario, you could take a look at Microsoft Store for business and line of business (LOB) apps.
Designed for organizations, Microsoft Store for Business and Microsoft Store for Education give IT decision-makers and administrators in businesses or schools a flexible way to find, acquire, manage, and distribute free and paid apps in select markets to Windows 10 devices in volume.
Another choice is that you could use Microsoft Endpoint Configuration Manager and Microsoft Intune to distribute your LOB app.