I hope that it is intended that Roon can be controlled from both PC, tablet and mobile phone. With the option to choose audio output from all of these??
As you can se from 2. above screenshot, Roon server is connected to SMSL USB AUDIO. But it is not possible to initialize this USB DAC, see 3. picture above
Installing drivers on your pc remote are not going to make it work if it’s attached to a QNAP core. It’s just installing driver for that pc only . The problem you have is usb audio on the QNAP likely a kernel issue with the DAC and what Linux kernel QNAP are using. Your better using a bridge of some kind to the SMSL such as another pc or raspberry pi if you have one or a low cost streamer.
As above what matters is the relationship between the DAC and the device it is plugged into. I think QNAP ended that capability a while back. I may be wrong but I think you have to have a bridge device for Roon with the core on a QNAP NAS.