None of my MKV movies won't play on ATV4K


I’m a new member and a fresh owner of ATV4K and Infuse 5 Pro.
My current setup is DS216play synology nas+ATV4K+LG OLED. When I try to play any .mkv movie from synology, I get “an error occurred loading this content”.
Please help and tell me what can I do to resolve this?