I'm not sure I understand your problem? What does being on a private network have to do with not being able to use the Sketch JS API? It comes with Sketch. All you need to do to use it is with this bit of code:
let sketch = require('sketch')
If you are able to share more of your code with how you are getting the
MSImmutableLayerGroup then I can give some more specific feedback.
If you really don't want to use the JS API you can use the native methods but I don't recommend it:
let nativeGroup = <MSImmuatableLayerGroup: >
let nativeArtboard = nativeGroup.parentArtboard()
Turns out the native method is the same as the JS method: https://github.com/sketch-hq/SketchAPI/blob/9ed6b0db077fc31b864a8015d2bf46f76bb4287e/Source/dom/layers/Layer.js#L91