How to enable WDR3600 IP passthrough mode to connect to AT&T gateway device?

I only see this i the manual:

VPN - VPN Passthrough must be enabled if you want to allow VPN tunnels using VPN protocols to pass through the Router.

PPTP Passthrough - Point-to-Point Tunneling Protocol (PPTP) allows the Point-to-Point Protocol (PPP) to be tunneled through an IP network. To allow PPTP tunnels to pass through the Router, click Enable.

L2TP Passthrough - Layer Two Tunneling Protocol (L2TP) is the method used to enable Point-to-Point sessions via the Internet on the Layer Two level. To allow L2TP tunnels to pass through the Router, click Enable.

IPSec Passthrough - Internet Protocol security (IPSec) is a suite of protocols for ensuring private, secure communications over Internet Protocol (IP) networks, through the use of cryptographic security services. To allow IPSec tunnels to pass through the Router, click Enable