Within the Plex addon, under your username drop-down → Settings → Video, I set everything to Original and ticked all the boxes.
Grabbed this random screen-shot from Google images…so ignore the values.
You were looking at the kodi settings for the add-on. I can see how this would be confusing as normally that selection would take you to the actual settings for an add-on. For this particular one you must open the app and then click on your username/email at the top right of the screen which will give you an option to go to the actual settings menu (which is the one TechZ is showing above).
Thanks for that , found the settings page .
Did it fix your playback issue? 