GPGPU HD Station HDMI 4K output

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Yes, Qnap have removed support for Plex via HD Station. I would suggest streaming via DLNA. You can get a cheap Android TV stick such as Xiaomi TV Stick https://amzn.to/3FpEgcr
Video decoder
AV1, VP9, H.265, H.264, MPEG-2, MPEG-1
Video Format
MKV, MPG, MPEG, DAT, AVI, MOV, ISO, MP4, RM
Audio decoder
DTS HD, Dolby Atmos
Audio Format
MP3, AAC, RM, FLAC
Image decoder
JPG, BMP, GIF, PNG

Alternatively Nvidia Shield TV https://amzn.to/3F8t3MU
4K HDR Ready
Dolby Vision HDR and HDR10

AI-enhanced upscaling for up to 4K 30 FPS
Up to 4K HDR playback at 60 FPS (H.265/HEVC)
Up to 4K playback at 60 FPS (VP8, VP9, H.264, MPEG1/2)
Up to 1080p playback at 60 FPS (H.263, MJPEG, MPEG4, WMV9/VC1)
Format/Container support: Xvid/ DivX/ASF/AVI/MKV/MOV/M2TS/MPEG-TS/MP4/WEB-M


Shield TV Pro https://amzn.to/3MUV38R
4K HDR Ready
Dolby Vision HDR and HDR10

AI-enhanced upscaling for up to 4K 60 FPS
Up to 4K HDR playback at 60 FPS (H.265/HEVC)
Up to 4K playback at 60 FPS (VP8, VP9, H.264, MPEG1/2)
Up to 1080p playback at 60 FPS (H.263, MJPEG, MPEG4, WMV9/VC1)
Format/Container support: Xvid/ DivX/ASF/AVI/MKV/MOV/M2TS/MPEG-TS/MP4/WEB-M


You can link all of these with a NAS you have.