Plex Buffer on Local Network Direct Play (Even on Small Files)

I have problem with my Plex. My Plex Server is installed on Synology Ds920+ and connected to my router. And My Nvidia Shield is also connected to router. (Cat7 cable and router supports gigabit connection) But even some video files that says requiring 18.5mbps buffer in every 5 seconds. And you can see in the pictures I uploaded that bandwidth is 560mbps already but video even not starting. I noticed that the problem is espacially with mp4 files.

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