I'm not familiar with that particular modem/router. However, you should be able to visit the internal configuration at
http://192.168.1.1/
From there, you'll want to look for some kind of Port Forwarding, where you need to direct the right port number to your specific computer (IP address).
Also, another reason that you may not be accessible from the internet, is because port 80 (default for web servers) is possibly blocked. I know many DSL (and cable) companies block certain ports including port 80.
If you find that you ARE being blocked at port 80, then you will need to specify an alternate port in Apache to 'listen' on.
- rp