Bug Can't set the aspect ratio to 16:9

So Im making a 2D game using UI, this game is for all the platforms and one of them is mobile. But I want the app to force the 16:9 aspect ratio. I just want black bars to appear when the mobile has a large screen. On the game window on unity I see the black bars (using the same resolution as the mobile), but when I make the build on mobile the app force the wide resolution making all stretch. Is there any way to force the aspec ratio?

Things that i tried:
-Setting the canvas scaler to Scañe with screen setting the reference resolution to 1920:1080, screen mathc modes that i tried --> Expand, match with height width
-Using a panel inside the canvas with the Aspect ratio fitter and setting it to fit in parent , aspec ratio 1.77777777 (16:9)
-Project settings, set the fullscreen mode to windowed and full screen.
-Tried all the differents options on the Supported aspect ratio section.
-Donwload the asset auto letterbox wich "forces" the camera to a certain aspect ratio, but maybe it doesnt work with UI.

Hope you can help me with that. And thank you in advance.

 

I don't have an answer for the issue, just a warning that this (black bars) can be a rejection reason for iOS.

Not sure about Android.