well friends... i'm german

and i do have the solution!!! But it's very difficult. So...
First of all the fire wall isn't the biggest problem, but you better switch it off. The main problem is the port. Only the Hoster must have one and that is the difficult thing because i don't know weather the ports are the same in german as in england.
You can set the ports on a router when you open the routersettings. On my computer i openet a prot with the number 2300-2310 and it worked. But those of you, who have internet over w-lan can't host. hosters must have a connection to the router over cabel. It doesn't mather if the joiners use w-lan or cabel too.
I'm sorry that i don't exaclty know what to do in your cases. But the hosters must open a port (router settings)!!!
best greetings from germany

lukas