You probably behind a firewall so your router is not accessible from the Internet. The ddns will resolve to your public IP and port.
About 72M and 150M, actually there is a reason. In 150M you need to use HT40 and use an extra channel. If the channels are too crowded and the wifi may automatically revert back to 72M. But you can force it to work on two channel, and it violates regulations.