Of course HDR10+ is available, my OP stated it plays in HDR10+ when starting movie from continue watching, tags only HDR10 though when playing from my library.
4K SDR resolution and set match content and frame rate to ON, 4:4:4 if available and make sure you have a quality certified HDMI cable, ATV 4K boxes can be finicky with cables.
Of course HDR10+ is available, my OP stated it plays in HDR10+ when starting movie from continue watching, tags only HDR10 though when playing from my library.