My Facebook app sends notifications with some information to its users with Notifications API. However, these notifications are only received on desktop, but not on mobile devices.
How can I reach all the mobile users community?
If you look at the Notifications API documentation: https://developers.facebook.com/docs/app-notifications/ it clearly states:
Note: Only apps on Facebook.com can use the Notifications API. Also these notifications are only surfaced on desktop version of Facebook.com.
There's no equivalent method for sending 'notifications' to mobile users - you could just email the users directly if you have their email address, which is available if they grant your app the email permission
If the actions you're trying to notify users about are user-generated you can also send Requests which are delivered to mobile devices when the app that sends them is a mobile app - see the requests documentation for iOS and Android