Summary of your issue

I was building "DlibDotNet.Native.Dnn" on the command line, referring to the web page "Tutorial for Windows". When I do a debug build, I get an error as shown below and the build stops.

net0.cpp
net1.cpp
net2.cpp
net3.cpp
net4.cpp
net5.cpp
output.cpp
adam.cpp
sgd.cpp
tensor.cpp
validation.cpp
D:\GitHub\DlibDotNet\src\DlibDotNet.Native.Dn

The Android AR sample needs the internet permission to be enabled. This hasn't been noticed previously, because the permission is added automatically as part of the debugging process. I only discovered it while preparing an apk and installing from a separate device. Xamarin doc: https://docs.microsoft.com/en-us/xamarin/android/troubleshooting/questions/android-internet