Yeah I know, Plex doesn’t support it … blah blah … use a converter …
I still haven’t seen a good explanation on why Plex doesn’t support DVD ISOs. Plex’s grandfather XBMC did support DVD ISOs so along the way it has been removed. Why?
ISO files are great for us who want the actual DVD intact, including all menus and extras. Converting them to mkv or whatever is not a good solution.
Transitioning between multiple .vobs when streaming also causes issues
Plex Media Centre was the only Plex client that handled ISO files, and even then only under specific setups (it needed to be running on the same physical machines as the server was installed on - limiting in to windows and OSX only)
Plex Media Centre and Plex Home Theatre are the only Plex clients which had anything to do with XMBC, both are long gone so what XMBC does is irrelevant to anything Plex does today.
If you want more context go search the old Archive Plex Media Centre forums and there is lots you can read.