If you could provide more specifics of your problem, you'd be more likely to get a specific solution. If I were to make a guess at this problem, I'd say that you not only have a standard router, but your "modem" is also a router, which is common fair for the most popular ISPs. This could explain why bypassing your router had no effect.
What is the make and model of your modem?
Also, what IP address are you giving to the players you'd like to connect? The one Fantasy grounds shows in the start screen is wrong if you're connected through a router.