I have a very simple home network with a computer and two routers connected in a chain. The first router simply acts as a kind of extension/wired access point and the second router connects to the WAN/internet.
DHCP is enabled on only the WAN router. Normal internet browsing and LAN functions are fine. L2TP passthrough is enabled on both routers, and I'm forwarding UDP ports 500, 1701 and 4500.
The problem is, no matter what combination of port forwarding I try, I'm unable to connect to a L2TP VPN when I'm using both routers. When I use only one router, I can connect fine.
The diagram below shows what combinations I've tried. What can I do to resolve this?