Popeye13
The Amazon Prime app has always supported FRM until an update last fall disabled it (subsequent updates have yet to re-enable FRM).
Prime's HDR tagging is often inaccurate (for several reasons).
As andyross suggested Amazon develops and publishes the Prime app (for all platforms, including RokuOS), so you'll need to contact Amazon CS about re-enabling FRM.
(Audio aside: You should configure Audio mode, Preferred Streaming Format, and Digital output format = Auto since the SS4K+ has no Dolby decoder. Note that ad insertions often cause audio/video related problems/issues, and you'll need to address this to both Roku CS and Amazon CS directly, and otherwise limit/avoid the issue by paying for ad-free subscription tiers if available)