Well, for starters, install Plex on your NAS from App Central. To do any of the following, you are also going to need to have WinSCP or another shell client running on your PC. Since WinSCP is free, just get that, and don't mess with anything else.... If you only have a MAC, well, I'm sorry, you'll have to do your own legwork to find an app like WinSCP for yourself.
There is only one model of NAS Asustor makes that can handle Real Time transcoding at the moment. That is the 700xT model. It has a real CPU in it and not a glorified cell phone CPU so it can handle some serious power compared to other models. The other models can't come close to handling the CPU demands required to do on-demand transcoding, so you want to reduce or eliminate the potential of it happening completely. To reduce the chance of on demand transcoding you need to convert all of your media from MKV's, AVI's, MOV's, MPG's, etc. to MP4 H264 with AAC audio in the first track and AC3 in the second if the original file has it. If you have DTS or something else you will need to edit the scripts.
This conversion is simple, as I have scripts that do this for you. Send me a PM for the FTP site to get the scripts. You make a shared folder called /misc and drop the scripts in my FTP into the folder. (So the full path is going to be /volume1/misc/Convert) Then you edit the cron file which I've got a few tuts already telling you how to do to put in the call for the conversion script and start dropping in the media. If you have a 64-bit NAS it's ready to run. If not, download the 32-bit version of ffmpeg from the original site (not App Central) and edit the ini file to point to the right versions of ffmpeg.
It takes the original media file, converts it, deletes the old file and puts the new file in a folder called /volume1/media/watch. From there you rename according to Plex's naming standards and drop it into your Plex Library according to media type. (Movie, TV show, etc.) You can get programs to run on PC or MAC to help with the naming. For movies I use FileBot on a PC, and for TV Shows I use TV Rename. I'm sure that these steps could be automated, Clinton Hall has some rathe nice scripts for his SickBeard/SickRage and SABnzb+ and in fact the scripts I have are based on his, but I've never gotten his scripts to work with my conversion scripts. Naming conventions for Plex can be found here:
https://support.plex.tv/hc/en-us/sectio ... edia-files You have to follow this pretty strictly to get the shows and movies matched up correctly.
These scripts are not mine. I edited scripts I got from the Plex forums, from one of the users there. They can take the existing srt encoded subtitles and make a stand alone copy along side the media. (MP4's don't support embedded srt's without them being hard coded into the stream, which defeats srt completely.)
I've already helped Jagstyles get this running on his 500x, and he and I did a lot of posting both int eh Tips forum and the Plex forum here at Asustor's forums. After you get Plex installed and running on your NAS and before you get too many streaming sessions going, you are going to want to install Python 2.7 from App Central if you haven't already. And you are also going to want to install PlexPy. This provides historical tracking of who, what, when, where and how your media has been used. It also can give you graphs showing a lot more details than Plex itself provides. You can get information on it here:
https://forums.plex.tv/discussion/16959 ... ng-program
This should be enough to get your started. Send me a PM if you want me to take a look at your set up or tweak things via a TeamViewer session. First one is free. After that I charge by the hour...