I was having trouble installing the codec because even though mx player told me to install the arm7 neon codec it didn't work. So instead I tried the AIO version and that worked fine for some reason.

Step 1. Download the AIO codec

Step 2. Send file from your Android device to mibox using Bluetooth

Step 3. Go to settings > decoder > custom codec in mx player. Navigate to Bluetooth folder and select zip

Step 3b. alternatively you can copy paste your AIO file to the download folder and mx player will detect it automatically.

Step 4. Successfull installation

FYI this was for my international version 4k mibox which I bought from AliExpress.