Getting the Jarnsmida Pitmines Gateway after beating the story

So, I apparently made a mistake when going through the mines the first time I was there and missed that Mystic Gateway. It seems to be the only way I can access the Asgard Remnant that's in that area, which sucks because that's the last one I have to beat to get past it. Does anyone know how I can get back to that area once you've beaten the story?