[quote user="Artem bursuk"]LoadRootModelById()[/quote]
I don't see that call in your code snippet.
Each DGN file must be opened before its models are loaded. How do you open each file? What options do you choose when opening a V7 DGN file?
Keep in mind that a V7 file is not a model container: the file is the model. Equally, a V7 DGN file does not have a model ID.