When you subscribe to a premium channel, like Paramount+, through Roku, your subscription is with Roku, not with the channel, and typically you must view on a Roku device, usually through The Roku Channel.
If you want to view on devices other than Roku (phone, website, other streaming device) you should subscribe directly with the channel on its website. Then you can view on any device for which the channel has provided a dedicated channel app.
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Re: Accessing Paramount Plus from the ROKU mobile app.
So if you want to watch Paramount Plus on your iPhone or IPad ; cancel Paramount on Roku. Go to Apple TV app on your Roku and subscribe to Paramount Plus. Then you can watch on your Roku AND your Apple products if you go to AppleTV app?
For clarification, if you want to access a channel provider's content on devices other than a Roku device, we would recommend subscribing through the channel provider directly, and not through Roku Pay. If you subscribe through Roku Pay, you will only be able to access the channel provider's content on Roku devices.