Setup L2TP Router from behind NAT

I have an outside router Netgear R8300 (RTR1) that is connected to my service provider, I also have a Linksys DGL-4500 (RTR2).

RTR2 is connected from the WAN port to RTR1.
I’m trying to setup the WAN port on RTR2 to connect to an L2TP server but I cannot get it to connect.

I’m thinking NAT is causing the issue, but when I go to RTR1 and go to add port forwarding, I have no idea where to forward it to since I’m changing the WAN port on RTR2 to connect the the VPN server.