Port forwarding in the TP-Link TL-R402M Cable/DSL Router is quick and easy but successfully allowing incoming connections to your web server requires correct configuration of other devices such as a modem/router. If the router is linked to a modem, make sure port forwarding in the modem works well by connecting it directly to the computer first and test if your web server is accessible from outside. (See the tutorial on How To Forward Port 80 for a Web Server via ProLink H9200 ADSL Modem/Router)
If everything went well, connect your router and reconfigure the modem to forward port 80 to the router's IP address instead of the computer's. It will be the router this time who will do the final forwarding to the computer's IP address. This is very similar to a bucket brigade setup where a group of firemen stands in line and passes over the bucket hand to hand to the next fireman and finally the water inside it is thrown to the fire.
Now test your web server if it is accessible from outside.
Questions? Visit the forum at http://www.istorya.net/forums/index.php/topic,169782.0.html.