Once you checked the box “Enable DLNA/UPnP media server” on your Synology DiskStation’s Control Panel it will create three folders: music, photos and video
Put your videos on the folder called video (preferred videos encoded as MPEG4 – they have the extension ended in .mp4 or .m4v)
To watch your videos on your Android phone, you should use two programs (actually, you can use just one if you use MPEG4 videos)
To browse your DLNA server, install BubbleUPnP
You can also install the MX Player, a video player that opens almost all videos available.
Open BubbleUPnP and click on Library then select your DiskStation and browse the folders that are shared. Once you find an item that you would like to play, click on it and BubbleUPnP will ask you which video player do you want to use. You can choose MX Player.