QNAP HS-251+ / Video freeze Problem

Hi,
I have a Plex server installed on a NAS QNAP TS-809 Pro (8 disks)
I just buy for my room a QNAP HS-251+ with plex added on for play on TV through HDMI.
When I launch a video, it was just a pain, the video freeze every 2 seconds.

What do I need to do on plex server to check if all is OK ?
When I play the same video on iPhone Plex it’s OK, no freeze, no problems.

Thanks

Where is the support team for plex ?

For now ==> Send back the QNAP HS-251+ for money refund.
Next week if nobody answer ==> Close my subscription for Plex…