You can use the mobile app as a remote since it will only control one TV at a time. Otherwise, you need to either block the remote's IR signal from reaching one TV (not necessarily easy to do) or invest in two voice remotes that get paired to each TV, so can only control the one each is paired to.
If you're not interested in having your TV connected to the Internet, I'd probably factory reset it and set it up as a "dumb" TV (assuming that's still an option with Roku TVs) and have it power on to the security system's input. When you go through the setup, just say you don't have a network to connect to. I don't know how that would affect using a voice remote, but since that's the first thing it tries to do when you set it up, it should work if you go the two voice remote path. You wouldn't be able to use the mobile app to control it.
Roku Mobile App - Free for iOS® or Android™ | Roku
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