Yeah, on the strength of this thread I think it is unlikely that you can input digital audio output from a computer into the USB on the front panel. I think it is far more likely that the receiver uses that USB port to find and play compatible stored files.
So, that leaves us with the coaxial and optical inputs. They will work, but don’t support DSD if that is important to you.
Devices available now that can connect by Ethernet or Wi-Fi and output to coax/optical input include:
Raspberry Pi and HiFiBerry Digi+
This has optical and coax out, supports up to 192 kHz 24 bit and can be Wi-Fi with an adaptor on the Pi or the inbuilt Wi-Fi of the Pi3.
This has optical out (the output plug is small and can be problematic for some cables). You can buy one and configure a Linux OS (eg. Armbian) yourself or there is the SonicOrbitor SE. You can configure a Wi-Fi dongle in Armbian but the SOSE is Ethernet only.
Expensive but probably the best sounding Wi-Fi solution. It is RoonReady but the coaxial and optical outputs are limited to PCM. Note that the Aries Mini is not RoonReady and Auralic have said it never will be.