iPhone: Notificaton at due date

I’m using the native iOS app to manage my tasks.
Also I use the regular web based GUI of the task app inside Nextcloud.

Now, when I create a task in the iOS app with a due date & time, the task will show up as notification with a beep on my iPhone at the due date/time. Also I see the due date/time in the web based GUI.

When I create a task inn the web based GUI and set a due date & time there, the task shows up in the iOS app, but the iOS app won’t recognize the due date/time and no notification shows up.

What property do I have to set in the web based tasks app in order to create a notification on my iPhone at the due date/time?

Thanks!