SharePoint's "Mobile Browser View" needs to be deactivated in order for Prime to apply its mobile responsiveness. Please follow the steps below to deactivate SharePoint's out of the box feature.
1. Click the "Settings" gear icon in the SharePoint ribbon to expand the settings menu.
2. Click on the "Site Settings" menu option.
3. Under the Site Actions category, click on the "Manage Site Features" link.
4. Find the "Mobile Browser View" section and make sure that this feature is Deactivated. If there is no Deactivate button available, that means it is deactivated. Now mobile devices will use Prime's responsive design for your SharePoint environment.