Able to access my IPCAM under LAN Cable but failed via WIFI

My config first:
1. Router is Buffalo WHR-HP-G300N
2. IP Cam is Wansview NCM620W (supports LAN and WIFI)

My router has been configured DDNS and opened port 8004 for my IPCAM.
I am able to access my IPCAM where it is connected via LAN cable using my own domain name of DDNS, I also can access IPCAM via GPRS with my mobile phone app (IP Cam Viewer). However I cannot access my IPCAM using DDNS when it is connected to my router via WIFI with my mobile phone app and PC.

So what is the difference between LAN cable and Wifi connection of my router with my IP Cam?? Thanks!