iPod 6th gen connect to DAC via USB

Hi there, I modded my iPod 6th to take the benefit of using Rockbox for 1Tb storage. However, when I tried to connect it with the Oppo HA2-SE DAC, it get into a USB mode as if I am transferring songs into it and I can’t play any song that can be through the USB. Would you know if there is a workaround? Thanks