The Roku app on mobile devices is primarily an alternate remote control for Roku devices and TVs.
Streaming rights for televisions, desktop computers, streaming devices, and mobile devices are all licensed separately. You can view Roku's own The Roku Channel via their mobile app, but not other channels for which Roku does not have the right to stream to mobile devices.
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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Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X | Streaming Stick 3600