Hello. My name is ***** ***** I will be assisting you today.
The server has Internet access briefly and then stops and there is not Internet access at all?
Can you ping the gateway on the server?
Anything else installed besides Exchange? DNS, DHCP, ADUC?
In the Hyper V settings for the Exchange server, can you remove the network card and then add in a new one?
You have another 2012R2 server on the network?
Export below 2 registry entries on a working 2012 R2 server. (right click on the folder --> export)
then move the 2 exported registry files to the Exchange server and double click on them to import.
Then open a command prompt and run netsh winsock reset followed by a restart
Can you ping gateway now still?