Here is a post on 10 iPhone Apps that Remind You to Pray via A Catholic Life.
I once downloaded an app like this. Problem was once I set it to St. Paul mode* I couldn’t turn off the constant notifications.
Another app I downloaded never reminded me to fast. I guess it didn’t have a fast processor in it.
Joking aside, the app that help me pray are the Liturgy of the Hour’s apps. Very helpful for me to get into the rhythm of praying throughout the day. I use iBreviary and Universalis. There is also the Divine Office app which is both text and audio, but I have no direct experience with this app.
iBreviary and Divine Office are also available on Android.
*(1 Thes 5:17)