So I have connected my old nas to a NordVPN server (openvpn)
And than I pointed pihole (docker) upstream to that nas with the vpn connection (I added the nas ip in the pihole upstream) and ...guess what. It is working, no dns leak when testing.
So I guess it’s a communication problem when connected on the same machine. I need to figure out how to let the nas communicate with docker on the same machine.