Well...first of all, is your ISP allowing access to port 53 from the outside world? You can check this by trying to connect to it from the outside world using nslookup:
> server 172.30.0.5
Default server: 172.30.0.5
Change the IP I listed there (172.30.0.5) to whatever your home IP is (the one that the rest of the world sees, not the firewalled ones. If you can connect and do queries against that DNS server, then the problem is very likely with your hosting service. If not, then you know that either DNS connections are blocked by your ISP, or you have not configured your DNS server properly.