Welcome to the Community 
The easiest way to do this is to use Styles.
This is the right methodology. Each Style has multiple settings, which is why, when you changed to unnumbered chapters, it appeared to add the numbered chapters as a sub-level. You can change these settings, although we usually recommend creating your own Style, tailored to your specific requirements.
For information on how to do this, take a look at these threads which discuss the topic:
Creating your own Style: Change Chapter Titles
Changing the hierarchy level: All Chapters stuck under preface
I hope this helps. Let me know how it goes.