Just got my Asus520GU and I've been reading tutorials on flashing the device.
The tutorial I followed suggested the easiest way was to first load dd-wrt via the firmware recovery mode and then using dd's UI, to install tomato. dd-wrt loaded up fine and when I hit the admin page I was asked to set a new password. Did that then navigated to the firmware upgrade area of wrt and, following the tutorial, renamed the tomato extension to .bin and flashed the device through the UI.
When the notification was presented that the install was completed I reloaded 192.168.1.1 only to be presented with a password screen that STILL had the title 'dd-wrt' I thought it was wrong and attempted to log in with tomato's credentials (root/admin) without any luck. I tried my password I set, the defaults, random combinations, caps, you name it - I wasn't getting in.
Next up instead of flashing the device with dd-wrt in recovery mode I selected tomato's image and flashed from recovery mode. Got a success message and again attempted to hit the admin panel. Same password prompts and unknown credentials. I tried rebooting the router and tried again but no luck!
Help me out guys I'm pretty technically inclined so if there's more advanced troubleshooting I could be doing I'm all ears. Thanks!