To do this, you can only use the BNC port of the D2 (not the USB port - which requires newer models of Lumin).
If your DAC has a coaxial input (RCA instead of BNC), you need to use a 1.5m BNC to RCA 75ohm coaxial cable.
In Lumin app, turn on digital output, set MQA mode to MQA Core, DSD mode to DoP (if your DAC supports DoP over SPDIF) or PCM.