“Above a river of lava” is the hint that pretty much reveals its whereabouts. Take a look at the map and the giant volcano. In its southeastern part, there is a small river of lava. If you zoom in additionally, you can also spot some brown colored structures. Head to the one that is a bit towards the north.
While in the area, you can spot the river of lava going down the mountain. It’s almost as if it goes down in the direction of our skyshard. The brown structure from the map is actually a Dwemer ruin. It hides our shard on a platform, almost above the lava.
Almost above the lava.
Sand Kingdom Moons - Super Mario Odyssey Power Moons