To config VPN passthrough follow all steps on your router
1. If your equipment supports NAT-T (NAT Traversal), turn it on.
2. Turn on port 1723 for PPTP VPN's- used for PPTP control
Turn on protocol 47 GRE for PPTP VPN's- used for PPTP control
Turn on port 1701 UDP for PPTP VPN's- used for L2TP
3. By default the router's firewall is configured to drop (delete) ICMP packets sent from outside your network to the WAN port. Your VPN may require the ICMP packets. To accept them:
(By default Select WAN Setup > Advanced > Respond to Ping on Internet Port.)
4. If you suspect your broadband device may not allow PPTP VPN traffic by design, contact the support team at your ISP or the device's manufacturer to verify this and see if there is a solution.
5. Your router firmware may need to be upgraded, you can go to the manufacturer's website and find the instructions on how to download and install the firmware.