In my case the level difference wasn’t too much (2 dB or so), but I like to have the same level going to my DAW and the uTrack 24. As far as I remember, I placed two resistors in series with the tip and ring of the TRS jack, connected to the (balanced) direct out of my mixer.
Combined with the input impedance of the uTrack24, they give the correct attenuation.
The resistors I used had a relative low value, in the order of 1 K.ohm, but you will have to find the correct value for your mixer. You can build the resistors in the TRS jack, if you use small ones (1/10 Watt or so.)
15 dB is a lot! So I suppose that you will need a ‘real’ voltage divider: like this:
1 resistor between tip and the wire going to the uTrack24
1 resistor between ring and the second wire going to the uTrack24
1 resistor between the two wires going to the uTrack24
A starting point would be 10 K.ohm for all resistors. But you may have to change the resistor connected between the two wires going to the uTack24, or maybe use a trimpot to determine the correct value first.
Maybe you can find a ‘technical’ person to help you with that?