[UG Cyber]

I am having issues setting up a Windows VPN Server for my work network.

I have setup the VPN through the windows network adapters window, then on a separate computer in the same network, I used windows to connect to the server using the internal IP address and it worked. However, as soon as i change it to the external IP address, it fails with error 619.
The server is behind a verizon Modem/Router. I have set the port forwarding to address the VPN server along with GRE.

192.168.1.***
PPTP
TCP Any -> 1723
GRE 	All Broadband Devices

I feel like i am missing something, but just can't figure it out.
Any help would be great!!

[UG Cyber]

If I provided my server with the DMZ, Would it help and what would be the best way to make sure it is secured?

There are A LOT of unsecured computers on our network (working on getting them secured, but its going to take a while) and I don't want to open a HUGE door with a welcome mat...