@sillygoose-
Did you "train" the new remote to use the right commend set to which your TV responds?
Roku menu path:
"Settings -> Remotes & devices > Remotes"
Since you've already paired the remote to your Roku, arrow down to select the new remote.
Now select "Set up remote for TV control"
.. And follow the prompts
Roku Community Streaming Expert
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X