To reliably receive notifications on your Android device, mediteo must be allowed to run in the background. This permission is requested when you first open the app. Depending on the manufacturer of your Android device, different settings may also be necessary to receive regular reminders. 

For OnePlus, the following points apply in particular:

Enable developer options (if necessary): Settings → About phone → Build number (or MIUI version) → tap seven times in a row until a message appears stating that developer options have been enabled.

Settings → Developer options → Apps section → Suspend cached apps → Off (Note: This requires you to restart your mobile phone).

In addition, the following settings should be checked:

Run the app without battery restrictions:
Settings → Apps → App management/Show all apps → mediteo → Battery usage/App battery usage → No restrictions/Allow background activities.

Settings → Apps → App management/Show all apps → mediteo → Manage app when not in use → Off.