Cormini
Reading on Nintendo.com, it appears you may have a problem with the firewall in your router. Perhaps either updating it or manually assigning an IP to your DS may help with this problem.
I tried manually assigning an IP to my DS but I know nothing about firewalls so that might be the problem. problem is that I don't know how to fix it and it might be password protected and I let another person do the password and I don't have it it got lost. Ill have to ask the person. Maybe it might fix it's self.