Is your mother living in your household? You can only control her Roku via the Roku app on your phone if your phone and her Roku are both connected to the same home network.
But note the app can control the Roku but does not mirror her Roku screen. Unless you are where you can see her tv screen, although you could set her Roku to the Netflix app via your phone, you wouldn't be able to see the Netflix screen to select and play a movie.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Do not ask me support questions via private message -- post them publicly to the Community where others may benefit as well.
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