I have a dyndns account and have the credentials on my home router. I also use port forwarding for remote desktop which works and port 80 to the same machine. I have iis on that machine running a small webpage.
When I am on my home network I can go to my dyndns account and view the page. If I try to do this from work, I get nothing. I can remote desktop into that computer from the dyndns name at work, but it won't return a ping.
So the dyndns entry works since I can get there with remote desktop, it won't return a ping or serve up a webpage unless on my home network. I would have thought that even though I was on my home network, the dyndns entry would look like an external connection so it would have been a good test.
I did setup the firewall settings to allow the remote desktop and allow http (which allows me to get there from inside). I am not an expert, but I knew enough to get this far. (To save time, please assume that I have already tried the obvious.)