The power, volume, and mute buttons are completely separate from the Roku controls, so it's not unusual that they would work while the others don't. For the others to work, the remote must be successfully paired to your Roku device. What's the model number of your Roku device written on it somewhere?
You've probably seen or tried these things, but just in case:
How to set up your Roku® voice remote | Official Roku Support
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