Hi!

I am trying to set up VPN on a TP Link Archer C6 v2.0 router. It was working previously, but after a change in the network configuration, it does not anymore.

Original setup:

Internet - (WAN) ISP router (LAN) - (WAN) TP Link router

In this setup, VPN was working perfectly, I had port forwarding set up in the ISP router, and I could access the VPN server on TP link router via internet.
The problem was, the ISP router does not allow setting up static routes, which meant that I could not access the TP Link router's subnet from the ISP router, which I want to, since I have a NAS running there.

Because of this, I switched to setup number 2:

Internet - (WAN) ISP router (LAN) - (LAN) TP Link router
So the only difference is now they are connected LAN to LAN, TP Link router being the DHCP server. Everything works, devices can reach other on both sides, everything perfect. The only difference is that even after I updated the port forwarding target to the TP Link router's new IP, I cannot connect to the VPN.

Here is what I tried:
- I thought maybe I botched the port forwarding somehow, so I changed the port forwarding target to my PC and left everything else intact, I fired up wireshark, and I checked out the traffic arriving from that port. Every packet arrives, it works perfectly.
- Then, I thought that maybe the TP Link router does not allow VPN connections from its LAN port. Being connected to the local home network, I started the VPN connection with the target being set to the TP link router's local address. It also worked, so this is ruled out, too.

Maybe it does not allow connections from different IP subnets, but only on its LAN port? Why did it work on the WAN port?

Again, I want to point out that everything else works, every device can reach each other and the internet, too.

VPN type is OpenVPN and I tried both UDP and TCP protocols, no luck.
I've also read the manuals, but it does not mention anything related to the problem.

Do you have any idea? Thank you in advance.