Hey there,
Serviio is a great DNLA. To say more, I think it is the best one, however, I am experiencing one strange issue with it on my D-Link 327L NAS.
For some reason, Serviio doesn't index the 4k video. I have checked the official Windows version and it worked great for the same file, however, the NAS version just didn't index it. One of the possible reasons - hardware requirements, but at the same time, the Plex Server did index and successfully stream it to my LG 4k TV. I played a lot with Serviio versions and DLNA profiles and nothing helped so my next and last guess - there is an issue in the exact NAS repack version.
@vtverdohleb if it possible, could you please take a look at that. I am running latest available Serviio version (1.9). Add-on package version is 1.07.
Searching Serviio forum also didn't help. People there claim this to be a Profile issue, but I drop that assumption by checking the Windows version with the same profile settings and 4k files were successfully indexed. Plex successfully plays the 4k, so for now I am running 2 DLNAs which is not quite good. Besides, I do not like Plex