Default Ubuntu configuration doesn't have any firewalls.I don't believe Apache is the problem here (if it responds, it is working). You should double check your network (router configuration, network configurations).I would first check your Ubuntu box for the IP it is configured to use:
eth0 (or something like that sould have a correct ip address (10.0.0.3 or whatever you said is your lan ip)
Then I would ping Ubuntu box. First from another machine in the same network. If it works, I would try http requests (from Firefox). If it works, then Ubuntu box is responding correctly. I would then test connecting from internet. If it doesn't respond, you might have misconfigured your router (probably NAT or Firewall rules).
You could also try traceroute 10.0.0.3 (from inside of your network, to see which routers are on the way of your request).