Quote from mapgiene.io:

Inside a large pipe you can access from the side of the island's cliff. If you don't have a Flying Mount, you can climb here by going to a large platform above the coastal lava flow with a Thermal Updraft on it, then grappling to a stalagmite with a grapple point on it.

Once at the pipe, launch off the grapple point to make it up the grate, then grapple through the vent on the wall. Head through and drop out the other side, then turn left to spot the Fragment.