Hi @Kevin_Z, exactly I am not able to access VPN after enabling UDP port 1701. I created an environment for C80 testing. Yes, my configuration was:

ISP modem router---C80---VPN server

Because I could not modify the "ISP modem router" settings, I put my old TP-Link TL-WR543G between ISP and C80. So my configuration is now:

ISP modem router---TL-WR543G---C80---VPN server

So I have full control over C80 from inside/LAN and outside/WAN.

I opened UDP ports 500, 4500 and 1701 and TCP ports 5001, 6690 and 5006 on C80 and tried to scan ports.
I used https://play.google.com/store/apps/details?id=ua.com.streamsoft.pingtools, but as I read now, it is able to scan TCP ports only. As expected, TCP ports 5001, 6690 and 5006 were open and of course they are used by my applications and they works.

I tried to use https://www.ipvoid.com/udp-port-scan/ and it show all UDP ports 500, 4500 and 1701 are open. But it probably tells that they are open on router of my ISP.
Today I will install some UDP ports scanner and I will try to scan my C80 UDP WAN ports later from my TL-WR543G network.

Anyway as before, I was able to create L2TP VPN in LAN, but with no success from outside, from WAN of C80 (LAN of TL-WR543G).

 

Detailed info:

TL-WR543G

WAN reserved, static 10.5.36.19

LAN 192.168.0.1

Port forwarding UDP ports 500, 4500 and 1701 and TCP ports 5001, 6690 and 5006 to 192.168.0.2 (C80)

PC (here I created L2TP tunnel without success)

192.168.0.100 reserved, TL-WR543G network

Smartphone

192.168.0.3 reserved, TL-WR543G network

C80

WAN 192.168.0.2 reserved, TL-WR543G network

LAN 192.168.1.1

Port forwarding UDP ports 500, 4500 and 1701 and TCP ports 5001, 6690 and 5006 to 192.168.1.11 (VPN/NAS server)

PC (here I successfully created L2TP tunnel, but VPN/NAS was in the same network, so it was directly visible without C80's port forwarding)

192.168.1.100 reserved, C80 network

VPN/NAS

192.168.1.11 reserved, C80 network

 

Am I missing something? You can see the log in the attachment of the unsuccessfull try to connect to VPN via Windows VPN connection. I am also unable to connect to VPN via smartphone.

FYI - as I communicated with TP-Link support, for L2TP IPSEC VPN (over port 1701)

Advanced-> Security-> ALG, IPSEC Passthrough

must be disabled for external access from WAN with beta firmware they kindly provided to me. I hope that the new FW with fix of 1701 port forwarding will be available soon.

Hello did you figure this out? I’m getting the same error "Conflicting with existing port: . To avoid conflict, please enter another value." How to I resolve this? Thank you