Unable to play 4k content - buffering - Please help

Hi,

I have plex server installed in a Proliant N54L server that runs correctly with the last version.
I have tried playing 4k content directly from the Xbox One X (not transcoding, just playing in the original quality) and its not possible to run it, it is buffering all the time.

My local network connection is gigabit and CAT6 cable (i dont use wifi connection). I have tried connecting a laptop to the RJ45 cable that i use to connect the Xbox One X ( to test the speed) and its transferring files from the server at 90-100MB/s speed, so all seems to be OK in the local network speed.

How can i solve that problem?
I have a UE55JU7000 TV with plex client installed in the TV too, and i have the same problem with 4k content.

Thank you.