When you say "narration", are you speaking of descriptive audio that describes everything going on in a scene? If so, that has nothing to do with the Screen Reader. Press * on your remote while a video is playing and see if there's a choice of Audio tracks under "Accessibility & language".
Is your Roku device a model 2720 as appears with your post? I didn't think it supported the Apple TV app, so it's not clear if you're speaking of an Apple TV device or the Apple TV app on a Roku device.
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