Kodi has had a cease and desist order placed against the entire project for a number of things. As a result Asustor is no longer supporting Kodi on the platform at all. Asustor's announcement of this discontinued support can be found here:
viewtopic.php?f=4&t=9544 This affects more than just Asustor, I might add. It's across the board for all NAS manufacturers, and all platforms that were capable of running Kodi at all. This is a big blow to those who want to stream their own legally owned media!
Basically it means the only viable streaming methods left for any NAS or streaming at all is now Plex Media Server. You are in luck with the model you have, since I believe it is capable of HW transcoding (might need a Plex Pass for it, though.) Read some of the links in my signature for some tips and tricks to optimize your media and device for the best Plex Experience you can get.