You have the option of connecting your dedicated streamer to the DAC via USB, Optical, Coaxial, or I2S (only a few supports it, but the best in terms of SQ). Figure out what output options your stream has, connect it to the DAC, then, use the screen to select that input.
If he was using Roon then there must have been a roon server, if that was connected to the DAC via USB that would explain “streaming without a streamer” or, more accurately, the roon server was acting as “streamer” too.