You would then have to somehow let the Delta Chat client know that the TURN server exists. Currently there is no way to do this through the UI.
Apparently we are going to have a default TURN server for non-Chatmail accounts, so you don’t need to worry about setting up your own server.
However, an option to specify a custom server might get added later.
Also see Help testing upcoming Delta Chat release with calls 📞! - #19 by WofWca.