Pairing a remote can be one of the biggest pains with Roku devices. A few tricks I've learned over the years are:
Make sure you have new batteries in the remote; weak batteries won't cut it to pair
Disconnect the power from any other Roku devices so that they can't interfere in the process
Unplug the power to your router while you're trying to pair
My preferred method is have the power to the Roku disconnected, press and hold the pairing button on the remote until the light starts flashing, and then connect the power to the Roku. Place the remote down close to the Roku device and hope it pairs once the Roku restarts.
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