The first episode of The Fate of Atlantis - Fields of Elysium - starts with a quest called The Isu Beckon.
After installing the content and entering the game world via The Fate of Atlantis menu, you should have access to a fast travel point Gateway to the Lost City located near the Volcanic Islands.
If you cannot see The Fate of Atlantis button in the main menu, please make sure your content has been correctly installed.
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