I don’t think it is supported on a UI level to connect to your NAS remotely. Might be if you alter some scripts but I would wanna mention that there are some security risks involved when not done with some precaution. Reading your post I would rather advise going to the NAS remotely on your office computer (check security), and then be streaming from your office computer to your Volumio device.
What I would suggest as the easiest and safest workaround:
Connecting to your NAS as you would normally do on your office computer (please mind NAS security when it comes to remote access!!). Now connect your Volumio device to the same network as the office computer. Login through the browser UI (might be that you need to use the IP, depending on your office network) Depending on the OS you are running on that computer it might be that you can see the NAS now. If so, there you go! If not try streaming through AirPlay (Mac) for Windows try streaming software, you have paid ones for sure but some are also free if I remember correctly. Also see: volumio.github.io/docs/User_Man … lumio.html
Wifi connection from your Volumio device is required sometimes, I guess.
For Linux you can use pulseaudio-dlna to stream to the device (at least that’s how I use it on a Linux), with this method your device also doesn’t need wifi, but it might still be useful if your office network turns out to be ignorant so to say . Pulse Audio will limit your quality to CD equivalent sound, 16 bit 44.1khz or 48khz (a long time ago I used it so I’m not sure). The method explained worked for me with a rpi3 without Wifi.
Cloudbased services are interesting but it all goes and falls with the API…