1 Remove power from the Gateway.
2 Disconnect all your computers and the cable/DSL modem from the Gateway router.
3 Using a straight through Ethernet cable, connect the Ethernet Cable/DSL port on the rear of the Gateway to any one of the LAN ports.
4 Re-apply power to the Gateway. The Alert LED will flash as the Gateway starts up, and after approximately 30 seconds will start to flash more slowly (typically 2 seconds on, 2 seconds off). Once the Alert LED has started to flash slowly, remove power from the Gateway.
5 Remove the cable connecting the Cable/DSL port to the LAN port, and reconnect one of your computers to one of the Gateway LAN ports.
6 Re-apply power to the Gateway, and when the start-up sequence has completed, browse to:
and run the configuration wizard. You may need to restart your computer before you attempt this.
7 When the configuration wizard has completed, you may reconnect your network as it was before
Here on page 85 of the manual
3Com 3CRWE53172 User Manual Forgotten Password And Reset To Factory...