Hey,
Can you make sure all of the DLC episodes are installed on to your Xbox as a first step: https://support.ubi.com/en-GB/faqs/0...ed-on-Xbox-One
Could you give more info on what is preventing you from starting the next episode if this is definitely installed on your console?
To confirm did you play any of the quests from the DLC previously? If so can you remember the last quest you did?
Can you also provide a video to show that the DLC is installed, to show the map in game and your quest log screen to show which quests you have available.
Before doing this I would suggest uninstalling and reinstalling the DLC again.