Using the CameraStreamController, the The Echo Spot and FireTV Stick do not seem to work for camera streams with resolutions above 1080P. I could not find any documentation that suggests these devices have a maximum supported resolution for RTSP streams.
The Echo Show, FireTV 4K 3rd generation and FireTV 4K 2nd generation device all work with camera stream resolutions at least up to 4K (3840x2160).
Thanks, Robert