Networks

My infrastructure: 1 AD Server, 1 VPN Server ( 2 NICs - private and public), Charter Static IP address pointing to public NIC. Hardware- Cable modem attached to Linksys BEFW11S4 v4 Router. Router Setup- GRE protocol allowed by PPTP passthrough being Enabled. Open port 1723 TCP in
"Applications & Gaming" on the router page to point to private IP 192.168.1.x. 192.168.1.x included in the DMZ.

Routing and Remote Access
configured properly on VPN server via PPTP protocol. Within my network, my users are able to connect to VPN just fine via PPTP. When they are outside of the network, and using there own ISP from home, they cannot connect via PPTP. They all get VPN error 800. I had them enter my vpn public host IP, rather than using the name, but to no avail. I am a novice to VPN, and was never able to get it working externally. I checked the event log, and it keeps showing me Event I 20209, Source Rasman. "A connection between the VPN server and the VPN client 192.168.1.1 has been established, but the VPN connection cannot be completed." Im not sure why it shows me my Linksys router gateway IP within that event log message. Im totally lost in trying to set this up. Again, it works fine internally, but not externally. I even updated my firmware on my router to 1.52.03. Please help a VPN novice.