How to send this signal to a mixer?
Jom, this is an excellent opportunity to learn troubleshooting skills.
When troubleshooting, you work your way through step-by-step, considering possibilities and gathering data to rule them out or in. (Doctors work this way too -- I should know, I've had careers as an engineer and a doctor).
You've confirmed that your circuit puts out sound by connecting it to a HiFi amp, then you connected it to the ZMX52 at port 2/3 and, no sound. You tried a series capacitor because of excellent advice from @Bimpelrekkie, that your circuit puts out a strong DC level.
Have you considered the possibility that your first attempt, connecting it to ZMX52 without blocking the DC, damaged the input stage of the mixer? Rule that out by connecting another known good source to that same input and confirming there's output.
What are other possibilities that are not obvious? Could your circuit have an intermittent connection that came loose during your experimentation? Could the ZMX  input have a loose connection (those phone jacks are notorious for corroding)? Could the ZMX have much lower gain than your HiFi amp?  Rule these out. Think of other possibilities. When you solve it, you'll have learned something.