Can I use Airplay to mirror to a ViewBoard screen on a cross subnet?

Yes, please use vCastSender iOS and key-in the on-screen connecting PIN code.

Then, on the iOS device, open the Control Center and touch AirPlay Mirroring.  

You should see a device prefixed with “Cast-XXXX”, select it.  

Enter the “AirPlay Password” to mirror your iOS device to your ViewBoard.