How to restrict restservices between Mendix applications in the same Cloud

We have two applications, one which contains all the data, the other to present data to the user on a mobile platform. Both applications run on the same cloud (Azure Frankfurt). We want to restrict access of these rest-api's in app 1 to app 2. How can we restrict access in app 1. Which ip numbers do we have to use?
1 answers

You can request the information on the IP number used by your application from Mendix support.

However, this is a shared IP number, which is documented here:

You should consider other mechanisms for protecting your rest api’s.

regards, Fabian