I don't know about Disney Plus specifically, but when you subscribe to premium subscriptions through 3rd party systems like Roku, Amazon, Hulu, etc, your accounts are with that 3rd party acting as an agent for the channel, not with the channel itself. There maybe some exceptions but these are typically only viewable via the 3rd party service through which you subscribed. In Roku's case, many are viewed via The Roku Channel.
If you want to view on all platforms supported by that channel you typically must subscribe directly with the channel via its web page then use its dedicated app for each platform, authenticating via the credentials you established with the channel account.
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. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
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Thank you for posting this. It is very good information and in my case a reminder. If you sign via a streaming service (ie Roku, Amazon Prime) you often can't access the service via another platform. I found this to be true with Paramount+. After having signed up via Prime I was unable to stream on my Roku TV. By signing up directly with the Paramount+ website, I am now able to utilize the service (and full membership features) on all my devices including Roku tv.