what operating system do you use? also i am going to assume this is a G300NH v1. if you use windows xp you can tftp like so..
Power the router off for now. Make sure you are wired into the switch.
1) Set a static ip on the pc of 192.168.11.2 / 255.255.255.0 Gateway: 192.168.11.1
2) Open a command prompt and type this: arp -s 192.168.11.1 02-AA-BB-CC-DD-1A [enter]
3) After entering the static arp have the command line ready with this command (and you'll have to be in the dir where the firmware image is at or it won't work): tftp -i 192.168.11.1 put wzrhpg300nh-pro-v24sp2-17798-alpha.enc
4) Do not enter the TFTP command, just having it waiting for now, Plug in the power for the router and count to 10 seconds, then enter the TFTP command that you should have waiting in the command prompt.
5) You'll know if it transferred by the command prompt telling you so, give it about 5 minutes after is transfer was successful, it'll reset/program itself in this time. One it is complete the Red Diag LED should go off and then you can access it at 192.168.11.1