If you have a reddit app like Reddit is Fun, then it'll open in that app.
Also, the mobile site of reddit is pretty bad. I really recommend downloading RiF https://play.google.com/store/apps/details?id=com.andrewshu.android.reddit
I installed an app called Better Open With, and for some reason it solved this problem. I configured the app, but didn't really get to use it. VLC wasn't showing either, and now everything is there. It's not the best solution, but you might try it and see what happens.
Last version of MX Player is known to have issues. Delete that app and either downgrade or get another video player app such as VLC
Pick an older version of MX Player here: https://www.apkmirror.com/uploads/?q=mx-player