Humm... It's hard to say what the problem is but maybe try enabling remote administration to the router. If you can connect to the router outside of your network then it tell me that it's something with the port forwarding setup. (If you do setup remote Admin. make sure to turn it off when done.)
I logged on to my router and noticed (and forgot) that you'll need to enable "ICMP Echo Requests"in order for ping to work. Enable that and see if ping requests are being returned outside your network.