Microsoft provides details guide how to setup unified interface app for mobile and enabling offline capabilities. Please refer following Microsoft Document for setting up mobile capabilities for D365 CE.
https://docs.microsoft.com/en-us/dynamics365/mobile-app/overview
However even after you complete all the steps users may not be able to enable offline capabilities of mobile CRM app. The main reason for that issue is, users may not have their offline profile configured in their user record. To fix that issue, you have to set Mobile Offline Profile for respective users. Following image shows how setting up Mobile Offline Profile for user.
After setting offline profile users should refresh their D365 Mobile app. After refreshing they can use D365 offline capabilities.
