I can’t get a correct resolution when using Infuse to play videos from my Synology. When I play videos directly from the NAS with my TV or internet box, the quality is great, but when I use Infuse with my Apple TV, quality is bad.
Are you connecting to your Synology using UPnP or DLNA by chance? If so, the Synology may be enabling some of it’s optional transcoding features which can lead to lower quality.
If you are able to create a new share and connect using SMB or NFS, Infuse will be able to access the files directly and avoid any of Synology’s built-in media server/transcoding features.