Since you subscribe to a premium channel through a third party like Roku, your subscription is not with that channel, it is with that third party. Consequently the separate Paramount+ app does not recognize you.
You can only access Paramount+ through the provisions of that third party.
In Roku's case that means through The Roku Channel on a Roku device.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X