Full screen button not working on mobile

Hi, I have tried to search this problem but haven't found a solution.

I have this course that is being custom developed for mobile, so the storyline player is entirely off. Since we need the full screen button, I have implemented it with an image and a trigger that it activates (toggle Player.FullScreenMode)

You can see the image attached. The button that toggles the full screen is the one at the botton, centered.

It works just fine on desktop but on mobile it has no effect. Instead the full screen from the standard Storyline Player works on mobile, but the client does not want the standard player, so we need to make it work with the custom button.

Does anyone know anything abojut this? Thank you