PPTP VPN - LCP: timeout sending Config-Requests

Hi Everyone,
I’m using FPBX 13.0.131
I have installed webmin 1.791 and PPPd version 2.4.5.
I have configured a VPN connection to a Windows Server and as long as I have the Firewall set to disbaled, I can connect and pass traffic perfectly.
As soon as the firewall is enabled, I can no longer connect.
When the connection is established while the firewall is down, I can bring the firewall up and the connection remains live, so I assume it is the GRE failing.
I have gre0 and gretap0 set as trusted in the interfaces. As the connection is establishing, ppp0 is auto added into the trusted zone.
I have made a custom service for port 1723 also.

When tailing the messages log at establishment time, the following output arrives…
The synchronous pptp option is NOT activated
Sent control packet type is 1 'Start-Control-Connection-Request’
Received Start Control Connection Reply
Client connection established.
Sent control packet type is 7 'Outgoing-Call-Request’
Received Outgoing Call Reply.
Outgoing call established (call ID 0, peer’s call ID 40079).
pppd 2.4.5 started by root, uid 0
Using interface ppp0
Connect: ppp0 <–> /dev/pts/3
freepbx php: /sbin/ip6tables -A fpbxinterfaces -i ppp0 -j zone-trusted
freepbx php: /sbin/iptables -A fpbxinterfaces -i ppp0 -j zone-trusted
LCP: timeout sending Config-Requests
Connection terminated.
Modem hangup

Could anyone suggest what I’ve missed here?