From the TD-W8960N Wireless N ADSL2+ Modem Router User Guide, which you can download from their web site.
PPP IP extension: Select this option to get the public IP address from the PPP server to your PC, and the NAT and SPI Firewall will be closed. Sometimes you can think it as bridge while PPP dialing in the router. Itís a special feature deployed by some ISP. Unless your ISP specifically requires this setup, do not select it.