We need a little more information to give a definitive answer, but there's two kinds of remotes that come with Roku TVs. If it's a voice remote, it has to be paired with the TV and can only control one TV at a time. If it isn't, it works like any old fashioned remote that you have to point at the TV and can control any TV of the same brand. In addition, some manufacturers use the same codes as others, so a remote for an onn TV may also control TVs from other manufacturers. The situation gets even more complicated because some (if not all new) Roku voice remotes also have the ability to control TVs via IR (infrared; i.e., the old fashioned way).
So, assuming the old remote uses IR since it sounds like it isn't a voice remote, it will always control any onn TV. To pair the new remote: using the old remote, navigate to Settings->Remotes & devices->Remotes->Add a new remote and follow the prompts.
Before you try pairing it, make sure it isn't already paired. Just because the old remote works doesn't mean the new remote isn't working.
Once you pair the new remote, if it isn't already, you should be able to use either remote.
The above assumes that the old remote isn't a voice remote and the new one is.
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