A couple of things to check:
The remote must be on the internet the first time the app launches so it can register with DJI. You are probably doing this but it is a thing.
Make sure you are using the 'e' variant of the APK file that is for Enterprise hardware. That is available here:
https://support.dronesmadeeasy.com/hc/en-us/articles/8160429529876-Installation
Make sure your subscription level is appropriate for M3E use. The Pro plan is required for use with Enterprise hardware.
https://www.mapsmadeeasy.com/map_pilot/