I'll get straight to the point.
If you want to increase user engagement for your app, your app had better be good in the first place.
Assuming it is, try this:
Try sending the user reminders to use the app via local notifications. I made a video on Ionic Local Notifications here:
When the user opens the app, register several notifications for dates in the future:
-the next day
-3 days later
-5 days later
-7 days later
-a month later
The notification should ideally say something cute, funny, or something else to make the user want to use the app.
Many apps have notifications built right into the network effect, like Facebook. Facebook will try to send you a push notification if one of your friends message you.
Try to make your local notifications as compelling as a friend messaging you. That may be hard to do, but do your best and keep it ethical.
That's all for now,
Jami Bot