After you come back from looking at the river of fire, Thanos is stomping toward you, and he is NOT in a good mood. Pause and save the game; this is an extremely time-limited puzzle.
Man, Thanos is an animal! Wouldn't you love to put him in a cage?
Sounds like a good idea to me. Did you notice that he's standing in the area where you were just caged?
This is where you need to use that "activation code" you saw in the note from Xenia's box.
What am I doing with Xenia's box?
See the panel on the wall at the left? Zoom in on it and use that activation code to activate the cage. Quick, because you have only a few seconds!
The code is 72914. Tap it in, and Thanos is behind bars, where he belongs. He won't be there long, but it'll be long enough for you to get away.