Ok. So, to be clear....
When you are sitting at the computer that is running WAMP, and you put the EXTERNAL IP address in the browser, you can view the web server.
When you are sitting at a different computer on the same IP subnet (connected to the same router and/or switch) and you put the EXTERNAL IP address in the browser, you don't see the web server.
When you are sitting at a different computer that is NOT on the same IP subnet (not connected to the same router and/or switch) and you put the EXTERNAL IP address in the browser, you don't see the web server.
Have you forwarded port 80 in the router to the internal IP address of that computer? Does your ISP allow inbound access/traffic on port 80? How would you test this? Well, first, make sure your router doesn't have a firewall blocking inbound port 80 traffic. Make sure you properly forwarded port 80. Make sure your internal IP address is static (in the DHCP table). If you have done these things... then it might be an issue with port 80. You can test it by going into WAMP and changing the port used to... whatever... 8080. This is just for testing purposes anyway. Then, go back into the router, and everywhere you dealt with port 80, change it to 8080. Then try accessing the server.