Try using the button on the RC to start and stop the video and take photos. Read the manual about these options. To take 20mp photos you must have the video capture off, otherwise you'll capture a frame form the video, which is less resolution.
Report back the exact words about the cache alert. That may be due to your smart device having inadequate memory to record video to your device memory (iPad or Android). In that case you may have to delete some unneeded files to make room for video recording to your tablet, or you can simply disable recording to cache to get rid of the alert messages.