OK turns out I was looking in the wrong place!
You have to browse to just System (Not System.Net.Networkinformation) and there it is..
Also the .net API browser courtesy of Microsoft comes in handy https://docs.microsoft.com/en-gb/dotnet/api/system.net.networkinformation.networkinterface.getisnetworkavailable?view=netframework-4.7.1#System_Net_NetworkInformation_NetworkInterface_GetIsNetworkAvailable