4 figure problem with prior landlord (HELP)

My prior landlord whose house I moved out of last June is still holding my 1800 euro deposit. Stichting WOON has advised me that they may not be able to hold the landlord accountable because they may not be traceable. According to Woon there are multiple prior tenants who have brought complaints against this landlord but have been unsuccessful. I know this myself because both my roommates at the time submitted complaints.

While the people at WOON have told me the landlord cannot charge me extra for cleaning fees (as there were no official reports made before or after my time there, also I confirmed with an agent that the state was good when I handed the keys over.) The relative is now saying they want 500 for the cleaning fee. Mind you its been for months since I moved out and I've had to send them my energy and water bill multiple times because they keep asking for them.

I've never been in direct contact with the owner but only through some agent and an apparent relative of theirs on WeChat. I have the bank account number I made my rent payments to, a couple of names, and the whole address where I lived for nine months. My question is how do I hold this landlord accountable to properly paying my deposit back and stop playing ring around the rosie with them?