Is this device not at all compatible with Plex?
Is there any software to install so that it is possible to watch at say a LG tv?
I'm assuming you have installed Plex from the addons and updated the library and that your LG tv can see your server (either via Plex or just the drive (via DNLA or other)
And I'm assuming that Plex can see all your video files but when it comes to play them via your LG tv it comes up with an error (something like file is corrupt)
This is because of the file type (or codec) used and your DLink NAS does not have the capabilities to transcode (convert the file).
I've had similar issues with my 320L and have found that in these instances it's best to convert the file to one that doesn't need transcoding. Generally converting them to DVD (vob files) would give the easiest option but depends on what formats (file types/codecs) LG supports.
I've tried Handbrake some successful some not and I've tried changing the container (mkv) again some successful but others not. If the file is HD with AC3 or DTS (surround sound) I find converting them to DVD and rejoining the Vob files is the best option for me, I use ConvertXtoDVD and then VobMerge252 to join the files together to make 1 (vob) file.
HTH