I assume you mean the Roku unit is connected via HDMI (and maybe USB for power). You basically need to use two remotes. If you're using the remote for the Roku player connected to the TV, it can't control any of the TV functions. If you have "1-touch play" enabled on the Roku player then you can press a button on the Roku player's remote to get the TV to switch to the Roku player's input, but there's no way to get the TV to switch back to the antenna. Once you're watching antenna channels, you have to use the TV's remote to change channels. If you press a button on the player's remote the TV will switch back to the player.
You can have the TV always power on to "Live TV" and that should get it to power on to the last antenna channel watched, but the way Roku keeps blurring what "Live TV" means I can't be sure of that. Note that you'd have to use the TV's remote to turn on the TV. If you're pressing a button on the Roku player's remote to get the TV to turn on then the TV will switch to the player's input. Once the TV is on the player's input turning the TV off and on using the TV's remote will get it back to the antenna input.
The bottom line is that there's really no way to get around using two remotes. You can't change antenna channels on the TV using the player's remote.
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