I recently got xDSL from one of the major danish ISPs. I was very satisfied until I discovered that port 80 was closed. I looked high and low for a solution. Eventually I gave up and contacted the support, which confirmed my suspicion: the router had an firewall which blocked port 80.
The solution is actually quite straight forward:
wan node index 1
wan node filter incoming tcpip 256 256 256 256
wan node filter outgoing tcpip 256 256 256 256
wan node save
wan node filter incoming tcpip 256 256 256 256
wan node filter outgoing tcpip 256 256 256 256
wan node save
And now my blog is accessible for the outside world again.