Hi:
I don't have any Android device running an HttpServer, but I think you're asking about the general process of redirect a port from your router to a local network device.
Have you look if your router have some embedded dynamic client? Some of them have it, then you just had to register in, and configure the account in the router.
If your router haven't it, you can use for example no-ip.com, they have a computer client you can run from any of your computers in the network, to update your ip.
Then you have to redirect some port from your router to your android device (test you can access your android device from local network first to confirm there's no problem with it). I'm sure you can find the way of doing this searching google something like "redirect port <your-router-model>"
Note that, as you say in the post you've linked, probably you can't reach your public ip from your network because of your router... this need some configuration in your router. So try reach your public ip from some computer out of your local network, or with a mobile connection.