You have several viable options that I can think of, the right choice depending on your use case of course.
1) A mendix native mobile app is just like a browser an interface for your user to interact with your application server. You can have both navigation profiles within the same project out-of-the-box without the need for a communication layer.
2) Create separate autonomous apps, one with a mobile (frontend) focus, the other as a backend app. You can use Rest to process data and keep the databases in sync between the frontend and backend apps.
3) Same as 2, except look into leveraging the Mendix Datahub for requesting non-persistent data to display in the mobile app. If offline-first is a requirement this option is not a good fit.
IMHO If you are using Mendix for complete setup, the additional REST layer is redundant.