https://docs.mendix.com/howto/mobile/apis This needs PushQueue active for queued messages to work. How to enable it? "For sending queued messages to work, you need to ensure that the scheduled event PushQueue is active. For TAP environments, you need to explicitly activate the scheduled event in the Mendix Cloud Portal. For free apps, the scheduled events feature is not supported, so a queued message won’t be sent."
You need to enabled the schedule event. If you are using the Mendix cloud you should see a section under your environment details called "Model Options". The first option allows you to enable the schedule events or not.
The module should come with a schedule event called PushQueue, which you need to enable to make sure the messages get sent. If you are not deploying on Mendix Cloud then you can configure the schedule events in the m2ee like so: