Sounds like you have the port-forwarding, virtual server or application rules feature turned on...

The WBR-2310 can be configured as a virtual server so that remote users accessing Web or FTP services via the public IP address can be automatically redirected to local servers in the LAN (Local Area Network).
The WBR-2310 firewall feature filters out unrecognized packets to protect your LAN network so all computers networked with the WBR-2310 are invisible to the outside world. If you wish, you can make some of the LAN computers accessible from the Internet by enabling Virtual Server. Depending on the requested service, the WBR-2310 redirects the external service request to the appropriate server within the LAN network.
The WBR-2310 is also capable of port-redirection meaning incoming traffic to a particular port may be redirected to a different port on the server computer.
Each virtual service that is created will be listed at the bottom of the screen in the Virtual Servers List. There are
pre-defined virtual services already in the table. You may use them by enabling them and assigning the server IP to use that particular virtual service.
Some applications require multiple connections, such as Internet gaming, video conferencing, Internet telephony and others. These applications have difficulties working through NAT (Network Address Translation). Special Applications makes some of these applications work with the WBR-2310. If you need to run applications that require multiple connections, specify the port normally associated with an application in the “Trigger Port” field, select the protocol type as TCP or UDP, then enter the firewall (public) ports associated with the trigger port to open them for inbound traffic. This is probably screwing you up.
The WBR-2310 provides some predefined applications in the table on the bottom of the web page. Select the application you want to use and enable it.
A simple solution: Make your computer the DMZ, but make sure you are using a software firewall.