Normally, if you subscribe to a channel through a particular platform, you are restricted to watch through that platform. Typically, if you subscribe directly through the channel/provider, then you can use their app anywhere. I would suggest cancelling your subscription and subscribing directly through Acorn, then you would be able to view on Roku or any other device.
You can try using your acorn login (email/password) that you use on Roku and try logging into the app on your other TV and see if that works.
