I am assuming you have a Roku TV. Many channels provided by your TV provider are encrypted, meaning they can only be displayed with a decryption device in the connection, which is what the cable box does. There is nothing you can do if your provider isn't providing those channels in the clear. No TV has the necessary decryption equipment to display those channels.
Depending on your provider, there might be a channel in the Roku channel store that allows viewing all of your subscribed channels without requiring a cable box. Spectrum and Xfinity both offer one, and there might be others'
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