I recently dealt with Instagram videos manually. If you open the network inspector while the video is streaming, you’ll see two different mp4 files being fetched repeated. One is for the video and another is for the audio. Each request will have a different timestamp in the URL query parameters.

• Right click any of the mp4 requests and click “copy as curl”.
• paste the curl command into text editor.
• remove the timestamp query param
• add > outputA.mp4 to the end of the command
• execute the edited curl command

Repeat with the other URL and save it to outputB.mp4 instead. Then view both files to determine which is the audio and which is the video. There was a very basic ffmpeg command which would merge the files. I can never recall correct ffmpeg commands though.