How to hard re-set a RB250GS, RB750/750G back to factory default

• Unplug all LAN and power cables
• Locate the "Reset" button, it's at the back right beside the power, use a sharp object, press and hold  (make sure you can feel the press down!)
• Insert power cable, continue to hold until you see bunch of lights blinking  (please be patient on this process, it may take few minutes!)
• Unplug the power cable, continue to hold at least another 30 seconds before releasing the reset button

Generally, I use this method to hard reset most the routers, so far it's been working fine!