Authention Failure when putting NAS in private network.

Hello, Here is my freeradius enviroment. The freeradius is used for WIFI users access authentication. OS: Linux Enterprise Server 6.1 Radius: free radius server 2.1.11 Database: Mysql The freeradius server was put on internet. Sometime, the WIFI router need be put on the private network, which connect to Internet through a gateway. When we put the WIFI router on the private network, the WIFI user authentication always fail. Checked both the server and client side, we found the server is sending packet to WIFI router(NAS), but the packet didn't reach the WIFI router. The packet was dropped by the gateway. Can anyone advise a solution for this? Thanks! Tom