Unpaid housing deposit as a foreigner

What are the realistic and concrete ways of getting a deposit back as a foreigner from the EU?

A housing company has not paid my deposit back, and it's been over a year since I moved out. Any reasonable timespan for holding the deposit has passed, and it is their responsibility to pay it, yet they don't.

The customer service is nonexistent, they tell us that they can't do anything about the deposit since it's the landlord's or their colleague's responsibility, but they refuse to a) give out their contact details or b) figure out the situation and call us back. Conveniently, the colleague is also never available at the time of our call 😉

So in essence, the company is an abusive housing company that takes advantage of the housing market in Munich to fuck with people. The 50 deleted Google Reviews confirm that.

I need that money back. How can I get it?