Actually, when the wdr3600 is working as a router, all the ports are blocked inside the network. Which means, your network information is invisible from Internet. I did not get the clear purpose about block all the ports expect 80. Do you need to block all the outgoing information or incoming information? Here is an instruction on how to block website: http://www.tp-link.com/en/article/?faqid=158 orhttp://www.tp-link.com/en/article/?faqid=359

Did you test with ShieldsUP:
https://www.grc.com/x/ne.dll?bh0bkyd2

Maybe this can work, set a white rule and allow access to port 80 only