If you have installed the codec but are still experiencing issues:
Note: If the application doesn't restart automatically, close it completely from your running apps list and open it again. Troubleshooting: Codec Not Working Mx Player Pro Codec Armv8 Neon
Allowing high-bitrate video to play without stuttering. How to Install the MX Player Custom Codec (ARMv8 NEON) If you have installed the codec but are