I spent a really long time trying to unbrick my router after trying to upgrade it with an incorrect build of DD WRT. I finally figured out how to do it. Hopefully these instructions save you some time.
Disclaimer: I am not responsible for any damage to your device(s) or anything that goes wrong. Proceed at your own risk.
Do not try this unless these instructions don’t work.
Disconnect power and all cables.
Dismantle your router (4 screws on the bottom, then pry case open with a flathead screwdriver. Start at the back of the router).
Unhook all the antenna wires (rotate the connectors and pull up on the wire gently).
Pull the board out of the casing and flip it over. There will be one chip on the back (EON EN29LV640).
Take a paperclip and bend one end out. Touch that end to pins 3 and 4 so you are shorting them (pin one is where the dimple is on the chip).
Plug in power to the board while holding the end of the paperclip against the two pins.
Remove the paper clip after five seconds.
Plug an Ethernet cable from your computer into the board and navigate to 192.168.1.1. You should see the Management Mode page where you can upload a new firmware. Download the stock firmware from the Linksys website (just Google it). You can also install a DD-WRT build directly from this page (I haven’t tried it).
Upload the firmware and wait. You’ll get a confirmation page.
Wait five minutes and then restart the router.
Reconnect the antenna cables (look here for pictures of how it should look) and put the router back together
You’re Done! Enjoy your newly fixed router. Please comment if these instructions helped you out.