This helped. Thanks a lot.
By the way the name of that tunnel, which doesn't even show up on the Dune Sea map is "Bantha's Drift".
But.. Before that.. I can't believe I spent more than an hour on a completely futile endeavour of getting there, moving around, jumping cliffs, getting killed by Imperial Guards, dying again and again and .. for what? The mission itself only takes like 2 minutes?!