USB headsets do not use the on motherboard audio chips, like the Realtek. The OS should load drivers for the headset and it should show up as a separate option other than the realtek audio. What is the model number of your headset.
Also, does Roon recognize any endpoint on the remote PC, like the Realtek Audio? If not, then the issue might be anti-virus or firewall running on the remote computer is blocking the Roon Server from discovering audio devices on the remote machine.