ok, SS9 has H.265 playback disabled in the browser version. Most likely, this is due to the fact that different browsers on different systems support H.265 differently. But, the recommended solution: the Synology Surveillance Station Client works very well, and the SS9 interface looks identical to the browser version and has the same features. For mobile devices, H.265 viewing works on DS Cam. Unfortunately, Synology Surveillance Station Client does not have a Linux version available, but Wine runs it without a problem: https://davejansen.com/synology-surveillance-station-on-linux/