I don't know about AMC+ 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. Although there are a few exceptions, these are typically only viewable via the 3rd party service through which you subscribed. That means a subscription through Roku is viewed on Roku equipment
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.
A workaround that's been reported to work with some channels (no idea if AMC+ is one of them) is to try to log into your account on its website, using the email address for your Roku account. (You'll find this address, if needed, via Settings/System/About on your Roku.) When the login fails, use their "lost password" option and respond to the resulting email to establish a new password. Now you (and your daughter) MAY be able to log into the regular AMC+ app using your Roku email and this new password on platforms other than Roku.
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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