EDIT: Most importantly ensure you have installed this:
SFC2 Directory Server mini-patcher for D21 - I reccomend fully disabling all software firewalls and anti-virus software while playing on the Dynaverse.
2 - If you are behind a router (and I assume you are as you mentioned wireless, but your public IP address is on a cable network), then you must use the DMZ feature of your router or more securely use port forwarding to forward ports 2300-2400 TCP&UDP and 47624 TCP to the static LAN IP of the SFC client PC. (
http://support.microsoft.com/kb/240429) The router must accept pings from the WAN and its firewall and UPnP must be disabled. If you have not read the full users manual for your router (not the "quickstart" guide), then I suggest you obtain it and read it in full. The following site may also be of assistance:
http://www.portforward.com/3 - Read over the similar issues that others have encountered here in this forum:
The How-To's of the D24 - If you're still stuck post again in this thread with more details; your OS, your router make and model, what software firewalls and anti-virus software you are running...