Video Creating a video within application runtime and exporting it to device as MP4

Hello, my project is supposed to have a feature where you can record a video file which is supposed to be a little story the player creates with both his voice and a scene. First, the player builds the scene and then records the story told with this voice, and so far i have nailed this part just fine, but i have only managed to make it so the player is able to export the audio of his story but i have no idea of how to make him be able to export a video file in MP4 with both the printscreen of the scene built and his audio being played over it.

The player-built scene in question is a static image composed of 2D sprites, like a child's painting, and i need to make it so the application(game) is able to export a MP4 file where it is essentially playing a single frame (the screenshot of the player-built scene) for as long as it is needed for the story audio file to be fully heard.

I have already tried the Unity Recorder asset in Asset Store but it is limited to function inside the Editor only, my project also needs to support both Android and iOS devices.