In the app, select the fourth symbol at the bottom, which in my case is Devices, where the duration is displayed in large letters. Under 'Your Brush Head' under 'Check for wear date' you can set the duration of the brush head and you will then also receive a reminder. I think the Zanh brush is great and much better than the previous models!

Works automatically on the toothbrush display.

I don't use the app and it always appears automatically for me.