If the problem still persists with the firewall off, you may need to open ports on your router through the Port Forwarding feature.
Create a manual connection on your Nintendo DS and assign a static IP Address.
Open ports 1 through 65535 as both TCP and UDP.
Ensure the ports are forwarded to the same IP Address given to the Nintendo DS system during the manual setup.
If your router's security software (firewall) is turned on, it may block communication that is required for online gameplay. To fix this, try disabling the router's firewall*.
Most firewall settings are located under the "Advanced" or "Security" tab.