MediaRecorder captured on Chrome not playable on Mobile or Safari
I think your problem may be in the first line:
const mimeType = 'video/webm;codecs=H264'The container you are using is webm, which typically uses codecs VP8, VP9. H264 is a codec used in the mp4 container.
Chrome supports webm. Safari does not (and all iOS browsers are based on Safari - hence your mobile issue).
You say that run on Safari, this outputs a playable video. use ffprobe to see what codec/containers are outputted on Safari - I am guessing that there is a change in container/codec.
Since your video is h264, you must simply change the container to mp4, and it will play everywhere. This is a 'copy' from one container to the other, not a transcoding, but you'll still need ffmpeg :)
Here's a post that might help: Recording cross-platform (H.264?) videos using WebRTC MediaRecorder