I had the same problem. I think its caused by either the router's or the firewall you have installed on window's blocking something that is required for remote viewing. The easiest way around this is to use install Hamachi on both the client and server side and set the hamachi as a trusted network (5.0.0.0) in your windows firewall. This would bypass any router and windows firewall restrictions.
Also the good thing about this solution is Hamachi gives you a fixed IP per profile. That means you don't need to use dyndns anymore. The advantage of this is you can use the DigiNet client software remotely. Because the DigiNet Client software only allows entering an IP during setup and not a DNS address. Therefore Dyndns will not help.
Good luck
