There are a number of apps on iOS and Android that support Discourse, but at least on iOS none of them are very good. However, there is a way to add the hub as web app on your home screen and get push notifications to your device!
How to make a homescreen web app:
How to enable notifications:
That is how I personally use the Hub and it works pretty nicely.
I have to add / I personally use the official discourse app plus a paid version (single / one time fee, €4.99 ) of pushover. I get pings and all for notifications.