How to merge IDM part video files and PLAY them?

[Drashya]

I downloaded a video through IDM, but it is in various parts. Only the first part plays, other have filetypes .mp42 .mp43 .mp44 and so on. I used the Command Prompt (to join all the files with the first one, but the output video also doesn't play past the first part.

I tried changing their file extensions to .mp4 and then joining them, but it too didn't work. I also tried HJSplit tool, but that didn't work too.

Is there any way I can get these files to merge and play like a normal whole video, like IDM does at the end of every download?

Thanks in advance.