Stuck in Wolfenstein: The New Order

[Gingerbread Demon]

I'm slightly stuck here.

I got past the bit where you use the tunnel glider and there's a large metal grate with a bit missing. I jumped down there and got an enigma code and a golden Lunula (necklace) but now can't get back up. I know there has to be a way back and it's probably bloody obvious but I am missing it.

 

[Robert Maxwell]

Genuine question: do you just not know how to use search engines?

This is the kind of question game walkthroughs are for.