This app was mentioned in 6 comments, with an average of 1.33 upvotes
I use this app to convert them. I select convert to mp4 (even if it's mp4 already). Then I send the new mp4 to Whatsapp and it works. It also helps with webm's from 4chan (they're sent without sound) by converting them into mp4's.
I've also got a microSD USB OTG reader in my car (the one I bought is out of stock now, but similar to this). That way if something happens I can easily pull up the video on my phone, and I can use MediaConverter to trim it down and share it.
I use on my phone and chromebook: https://play.google.com/store/apps/details?id=com.AndroidA.MediaConverter&hl=en%5B%2Furl%5D
but also downloaded to try : https://play.google.com/store/apps/details?id=com.xvideostudio.videocompress&hl=en%5B%2Furl%5D
L'appli Media Converter devrait permettre de changer le bitrate, la définition et le codec.
I found this app that should do it, although I've never used it so can't say for sure.
use this Android App (which gives good quality videos) you can convert anything. https://play.google.com/store/apps/details?id=com.AndroidA.MediaConverter&hl=en