If you have an AMC+ account and you want to watch on our AMC+ app, you just need to download it to your device. The main potential issue is that you accidentally go to the Prime Video AMC+ add-on channel. That's a different subscription type.
How to get the AMC+ app
- From the Fire TV Home screen
- Select Find or press the voice control button to search by voice
- Type or say 'amc'
- Amazon will show several results
- Choose the one that says simply AMC+ with the blue-ish logo (not the back-and-white logo, also not the one that says 'Prime Video Channels' on it)
Look for this when you choose the app:
And do not choose either of the ones below―these are both shortcuts to AMC+ within Prime Video Channels