@Kevin_Z oh, now it is clear to me. The router can connect to VPN server of the ISP. Am I correct? But in my case I use "Static IP" assigned by my ISP. In this case the port should not be reserved....

@felixk see also this topic: https://community.tp-link.com/en/home/forum/topic/223292

@Kevin_Z yes, as you wrote. My NAS is connected to the LAN port of the Archer C80 and I want to visit it from the outside/WAN network. I already configured ports and it is accessible from the WAN....

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...

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....