sounds like the problem is accessing the routers config, in general. the best option, sadly, would be to call the ISP, or refer to their website, and ask about hosting. my provider went off on me for trying to have a static ip. maybe yours is the same way?
you'll end up having to open the ftp port - which they wont like. another option - probably your best, is to use port 80 (http) for your ftp server. they cant block / restrict that, right? perhaps that will work, but NAT is a fickle beast. best of luck, keep posting.
also, check out www.dslreports.com - they have loads of info about getting around various ISP restrictions legally.