You subscribed through Amazon and so you can view on Amazon Firestick devices. Had you subscribed through Roku you could view on Roku but not Firestick.
This is true in general for most (all?) channel services that you subscribe to through third parties. Your subscription is with the third party. not with the channel so it must be viewed via the third party's platform.
If you subscribe to the channel service directly on the service's own website you can view that channel on any platform the service supports -- the channel's website and any dedicated apps the channel may have released for Android, iOS, or specific streaming platform.
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.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X