two factor authentication by android app for VPN NPS

local 2 factor authentication for Windows VPN by Android phone -DLL written in C++ to extend Windows server NPS (Network policy server) functionality -Android application that receives a Firebase Cloud Messaging request and sends an HTTP POST back to the servlet once a button is clicked. -Java servlet that receives the DLL request, sends a POST request to Android for the notification), receives an HTTP POST back from the Android and sends an HTTP POST to the dll file which will then approve of the 2nd authentication factor allowing the VPN connection to be established. Currently, the 1st authentication factor is available (user can log into VPN with correct password and username set by active directory). The servlet is partially built and is able to send a notification to the android application. DLL file code (example) is available but is too extensive and has to be simplified. No need for an extensive solution, minimum simple code will do as long as it works. I'm willing to have a Skype call to discuss things and share more data.