Android Push-Notifications not working?

I have the problem, that Push notifications on Android does not seem to work on my nextcloud-instance V17.05. I have alread read through different posts with similar issues that brought me to the command “/ocs/v2.php/cloud/capabilities” to check the capabilities of my server.
My problem is now, that the “notification” capability is not listed even if I am authenticated.
Is there a way to activate the notification-capabilities on server-side? How?
Thanks for your help-