How to change the default video player
Helix TV FireStick app comes with a built-in video player. It is good, but not good enough. You may encounter buffering issues with it. Therefore, I recommend using an external player, such as VLC or MX Player
Read: How to install MX Player on FireStick
Here is how to change the default video player in Helix IPTV app:
Open Settings from the main screen (cog icon in the top-right corner)

First, we need to add the external player. Click External Players

Click Add Player

Choose MX Player or any other player you wish to use
Note: You can add multiple players

Click Add to confirm

Go back to the Settings screen and click Player Selection

As you can see, Helix TV app on FireStick lets you choose a separate player for each category.

Click the category for which you wish to change the player (say Live TV) and choose the desired player (say MX Player)
Now, simply click the Back button on the bottom of the screen or press the back key on your remote for the changes to apply