I have the same problem: it is because my ISP doesn't allow loop-back. In other words, you can address every other device on the internet, apart from the one you are using as a gateway.
There is no easy solution. I can look at my web-site if I use a proxy (there are many of those to choose from), but I haven't found one that allows a port specification, at least not among the free ones, so I cannot see my webcams.
So, if I want to look at my webcams, I need to use their internal address 192.168.???.???:????. If I want to check that others can see it, I log into my neighbour's WiFi (this is not a hack, but a free WiFi share service provided by the ISP that we both use).
Without this, if you still have a modem and dial-up internet access, you can use this to log in with an alternative IP address and check your camera that way.
If you come across a proxy that supports port addressing, I shall be delighted to hear about it!