Cannot access Ubuntu server behind Huawei Router with port forwarding

I have an Ubuntu 12.10 server installed and have a static IP from my ISP. Pinging the IP results in success. I also have a Huawei Echo-Life HG-521 modem. I setup port forwarding with instruction from this site. I also checked this with the guys at Huawei. The problem is that when I enter my static IP in the browser, I always get stuck at the modem/router login interface.

Going to the IP address assigned to my Ubuntu server (on my network) I can see that nginx serves pages correctly. Another thing is that getting my modem/router interface is only possible via my local network. I asked a friend to try my static IP but he said he got a blank page. I think this means that the gateway is not accessible from the outside world.

Any idea why I might be having this problem? How should I troubleshoot this?

You never say which external port you are forwarding to the Ubuntu server, and you also never say what port your Ubuntu web server is running on. I'm going to assume port 80 and/or 443 for both cases.

It is highly likely that, in spite of port forwarding rules, the modem's web management page is parked on port 80 and intercepts traffic before forwarding rules take place. I have seen this exact scenario in lower end routers and modems. You will likely need to change the management port for the modem to something other than port 80. Port 8080 is a popular alternative.