Buffering on UHD Content

I’m having trouble with both of my Apple Tvs streaming UHD (high Bitrate) movies from my Synology 920 Nas. Both Apple TVs are connected to an FritzRepater 6000 - one per Cable and one via Wifi but only 2 meters apart with no walls in between. (wifi connection is shown to be 900Mbit in the router menu)

I tested all SMB Options and Buffering Options avaiable but my speedtest on both devices is always around 75Mbit and therfor buffering every few minutes

Last night i tried different cables and removing the Gigabit Repater in between but no significant changes. After i brought evenything back to the state before testing i got one speedtest with 600Mbit but this morning im back to 75Mbit.

Any ideas what the problem could be? Streaming to my madvr pc is working withput problems so i don’t think that the NAS is the Issue. Its connected via an USB 2.5Gbit Cable Adapter.

Thanks in advance