Plex recently announced trailer support had been added with the recent Xbox One app update, but for the life of me I cannot figure out how to enable this feature. On all other apps the option to enable trailers is very easy to find, but the settings on the Xbox One are pretty sparse, with no options to customise the app experience.
I've tried changing trailer settings on the media server (settings/web/player & settings/server/extras) but this only enables trailers through the web app on that server device.
Figured it out. I added a new film and the trailer icon appeared. I was using TMDB as the primary agent before, I think this has automatically been changed to Freebase since the update. Freebase had to be the primary agent, so I have deleted and re added my entire film library. The trailers now appear on the Xbox.