I’ve found that the most reliable free online method is using the Clever Online Video Repair tool. Here’s my step-by-step process to recover corrupted MP4 files or other video formats:
Go to repair.cleverfiles.com.
Upload your corrupted video file (up to 5GB).
Provide a sample video from the same device with similar settings.
Let the online tool analyze and repair your video.
Download the repaired file.