I believe you don't see the Isolated
property in Solution Explorer, because you manually generated the Interop assembly with tlmbimp.exe, then added it to the project.
Don't do that. Remove that existing reference, then add a new one, where you'd browse for the original unmanaged COM DLL. In Reference Manager, Click COM
at the left, click Browse
button, then browse for and select your unmanaged COM DLL. Visual Studio will do the rest.