Mesh missing from Lumen overview based on distance? Shadow issue.

I’m having some trouble with a large-scale masterplan model - not only are some parts of the model appearing black when reviewing in “Lumen Scene” - but other parts are completely missing? See below the same scene with the camera moving forward. Whole objects are being culled when at a certain distance. The issue is this is playing havoc with the shadows as vast areas that should be in shadow appear lit and then suddenly get a shadow. I am unsure which settings to adjust to fix this.