Players who own either or both of the Steeltown and Cult of the Holy Detonation DLCs, and have them installed will see a splash screen upon logging in the first time with a message that confirms that DLC content is active and available.
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With the DLC installed, as soon as your squad is level 9 you will receive a radio dispatch invitation to Steeltown as soon as you access the world map in the Kodiak. Likewise, before you get the call to head to Cheyenne Mountain for the Cult of the Holy Detonation content, you will need to be level 16 when accessing the world map in the Kodiak.