What is required to get a mobile app working on Quest?

I'm trying to port my launcher app from android over to Quest. I've followed https://developers.meta.com/horizon/resources/migrate-2d-app . I can build and debug-run the app on the quest. But that's as far as it gets. The quest (a Quest S3 in this case) shows only a black screen with 3 loading dots. I see in the logs that my app is running but the main activity (as well as the entry activity) is not showing. It is as if the quest is trying to run it as a 3D app instead of a 2D app. What special settings do I have to add to the manifest to ensure quest loads it as 2D app? (or rather, the main activity has to be 2D and a later activity switches then to 3D)