Online scanners usually don't support UDP scans, because they are a pain to perform.
If you are sure the port is open in your firewall and properly forwarded, but DS's still can't connect from outside your network, it may be your ISP blocking the port ; you can't do much about that, sadly.
Oh, and I was wondering ; do HyperGTS' DNS server respond with your local, or your public IP when someone tries to access gamestats2.gs.nintendowifi.net ?
That could be the source of the problem ; if it gives your local IP to machines connected from the Internet, that can't work.