Rise of the Tomb Raider survival cache
You can access that area of the building during and after the side mission "The Unlucky Ones" which has you rescuing some remnant prisoners. You climb up to the back of the building by way of the small island in your picture (near the cave icon). To get this side mission you first have to complete the story mission "Dangerous Territory" which has you explore the Soviet Prison in the northwest area of the map.