For the "Auto Stop" feature to work you need to own "Donut Dog Pro".

If you just bought Donut Dog Pro. Please go to the settings and turn on the „Auto Stop“ feature.

Now turn the "notifications" option off and after 5 seconds on again. 

Make sure that notifications are allowed in the Apps settings as well as in the settings from your iPhone.

If that does not work please restart the app, make sure that the settings are still correct and try again.

Other causes:

Also a known cause for some iPhones is multitasking (like playing Youtube videos for music).