Cannot get Apple TV + on Fire TV

For most models Fire TV (2nd generation or later), users should be able to download the Apple TV app from the on-device Fire TV Games & Apps Store (or how else it is named these days). Download apps on your Fire TV - Amazon Fire TV Support

Once you have downloaded the Apple TV app, you would need to sign in to the app, through the in-app Settings (sprockets wheel icon), with your Apple ID account that has the subscription linked to it.

I would think that a device would only show compatible apps. At this point, I think you’ll need assistance from a Fire TV expert, which I am not. Perhaps one of the other forum users knows more, or maybe elsewhere (Fire TV Support, Fire TV forums?). Good luck.