Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
That's the way it works when you subscribe to a premium channel through a third party like Roku, Firestick, etc. Had you subscribed through any other platform, you'd only be able to view it on that platform. A third party subscription is with that third party, not with the channel directly, so your third party credentials are not valid on any other platform.
As @RokuDanny-R already mentioned, if you intend to view on more than one platform, subscribe directly with the channel on its website. You'll then be able to use that channel's app on any platform supported by the channel.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T