[TilesetJsonLoader.cpp:810] Received status code 429 for tile content

That source file exists in cesium-native.

If the initial tileset load is failing with a 429, then subscribing to this event should be what you want:

But if individual tiles are failing with a 429, that’s not going to help. I don’t think there’s currently any way to be notified of those errors from the cesium-unreal side. It will require a change to cesium-native.