If you truly have a Roku Simple Remote (infrared (IR) only, no microphone button, no power, volume, or mute keys) then it cannot be paired with a Roku. IR remotes just work by pointing them at the front of ANY non-Streaming-Stick Roku device without being paired to it, as long as there is a clear unblocked line of sight from the remote to the front of the Roku device.
Be sure you have good batteries installed in the remote, in the correct orientation as as shown by the diagram in the battery compartment.
You can check if the remote is sending IR signals in the first place by pointing your remote at a digital camera, even one built into a phone or tablet, and pressing some remote buttons. If the remote is sending signals you should see them glow from the top end of the remote on the camera view screen.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X