And DLNA is mostly useful for when the "display" is unable to decode the movie.
Q5/10 Pro is more than capable to decode any movie and it offers features not available in any DLNA software - choosing audio track, choosing subtitle track, adjust timing etc. Plus better image quality!
And you have one dedicated CPU to do the job. Synology is smart enough, but it is well optimized for file transfer rather for decoding videos. And why not using a CPU build for this purpose but split the load between Q10 and Synology?
You're trying a feature that has more drawbacks than benefits. And it doesn't work in your case. And this is for the better, configure your system to use SMB or NFS and forget about DLNA.