Sounds like you have not set Port Forwarding on your router.
In order for your router to allow traffic into your private network you have to tell the router what ports are allowed to accept traffic. This is called port forwarding.
Basically you will have to port forward port 80 on your router, part of this process is to tell the router which PC on your private network should have requestes on port 80 sent to. This would be the PC running Apache (wamp )
Have a look at this site, its pretty good and probably even has specific instructions for you exact router.
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portforward.com]