Nowadays, antivirus solutions are designed to be on-line and Avast is not an exception.
You need to be on-line to active your license on an unmanaged solution and you need that your console will be on line on a managed solution.
On a off-line enviroment you lose a lot of features, not only the VPS update, Avast CyberCapture is basic for a correct protection or file reputation.
If you have an off-line enviroment, you need, at least, one on-line device to be a Local Update Server and deliver program and VPS updates.