You set up a new instance of Dynamics 365 for Customer Service.

1. User is able to login but can't see Case Records --> "Configure Mobile Settings on Case Entity Level"

2. Users can open cases but cannot see the subject of the case - “configure mobile settings at the field level within the case form

3. User reports that they cannot access the system from Dynamics 365 mobile app --> Configure a security role in the mobile permission set of the appropriate user

https://docs.microsoft.com/en-us/dynamics365/custom er-engagement/mobile-app/set-up-dynamics-365-for-phones-and-dynamics-365-for-tablets