@Sunshine I have just applied a new firmware from the TP-Link support engineers and it has fixed the problem. Thanks for everyone's help.

@woozle Thanks for your investigation. After switching off the "IPSec Passthrough" option, the UDP port 500 does start forwarding. (I'm testing on public network now. Last time I tried, I was testing...

@woozle I used the netcat command line tool to listen to the UDP ports in the VPN server then I sent messages to it via the ports to see if the VPN server can see the messages. The VPN server is a...

Hi I have a L2TP VPN server behind the router. UDP ports 4500 and 1701 can be forwarded to the VPN server but not the UDP port 500. I have tried Port Forwarding, UPnP and DMZ. I have enabled L2TP...