Hi Julian, unfortunately Runtime iOS SDK doesn't fully support GeoJSON, unlike the GeoJSONLayer API in Javascript. 🙁 Runtime does support very simple GeoJSON such as creating a geometry, but not much else. There are a few options:
1. Publish the GeoJSON file to ArcGIS Online and consume it as a feature layer. See the doc here.
2. Break the feature collection into separate geometries manually.
3. Use tools to convert it to other formats that Runtime supports.