iOS app and App Store Review Guidelines

Hi, I compiled my own version of Mattermost iOS application and App Store rejected my app for the reason that “We found that the usefulness of your app is limited by the minimal amount of content or features it includes. Specifically, its only feature is chat. We encourage you to review your app concept and incorporate different content and features that are in compliance with the App Store Review Guidelines.”

More exactly it is their guideline 4.2 Minimum Functionality

Has anybody encountered with similar issue and can you advice me the direction to move? Thank you.