In the later part of Atlas Fallen, you will pick up a quest called “The City of the Sun” and one of the objectives you get for it will task you with “dress as unnamed and enter the kiln room”. If you try to enter the Kiln Room without this, the guard will stop you from going inside. Here is what you need to do to get inside the room and how to get the disguise you need.
How To Dress As Unnamed
Unnamed is what you were at the very start of the game, so all you need to do is change the current armor that you have equipped back to the clothing that you had at the very start of the game. You should have an unnamed worker outfit that you can use to get inside the room. Feels like a fairly dull and almost pointless task but that is what you need to do to get inside the Kiln Room in Atlas Fallen.