@terziyski my VPN Still doesn't work. It was enabled by default. I also tried to disable it and configure again the port forwarding But no succ

@Atlasior TPLink AC1900 EU ver 1.0.

Same problem, still no solution?

Maybe the vendor har set it up internally to be able to access it remotely....

First post was i aug 2020, still no solution???

@Atlasior 

Hello, not sure if it helps, but it worked for me.

Problem is, you are trying to forward a port already used by your router, close the app who's using it, port will not be used by router, forward the port and then, open the app using the port. It worked for me, for Qbittorrent on a Archer C80 with the lastest firmware.

Good luck :)

There is nothing using a forwarding port at all. 1701 isn't in the list, and it can't be added as well.

I think TPLink is using it behind the scenes, whithout telling us....

Hi @Kevin_Z, I have the same problem. I found, that there is problem with External Port. I tried to add External Port as 1700 and it worked.

I tried to unplug WAN cable, reboot router, change ext. port from 1700 to 1701 while WAN cable was still disconnected, but with no success. Still same error:

Conflicting with existing port: 1701. To avoid conflict, please enter another value.

Are you able to simulate it in lab conditions? Can I help you with my environment?

Isn't this port 0x6A5 used by TP-Link ID, Tether, OneMesh, Smart Life Assistants IFTTT/Alexa or...?

Hi @Kevin_Z, yes I want to open the 1701 port for L2TP/IPSec VPN server on Synology NAS behind the router. I am not sure if it is possible, because I found another problem, that my Synology NAS DS220j do not recognize TP-Link Archer C80 via UPnP correctly and NAS can't configure it properly. I contacted Synology if they could add Archer C80 among known devices. Please, could you contact Synology to speed up things? Until I can't configure router I can't check if it is possible to run VPN server on different port. VPN ports configuration is not accessible while router is not configured properly. I am sad of it. You can see configuration of NAS VPN here: https://youtu.be/1aaqqw3eQac?t=489
Thank you for the information that you found the reason why the 1701 port is not available. I have the question. If port 1701 is reserved for the L2TP WAN connection, is it possible that Archer C80 could act as a VPN server? At least it would be fine if router administrators have option to change all reserved ports of router. Are these ports in use? What is the purpose of them? Does TP-Link have the opportunity to log in to router or is it used for Tether mobile application, TP-Link ID...?
I hope that the optimization future will come soon!