To reset the device (briefly depress the button for more than two seconds but less than ten seconds then release). Caution: If the reset button is pressed for more than ten seconds while powered on, the device will reset back to its factory default configuration
To reset the device to its factory default configuration (depress the button for more than ten seconds then release)
its not empty, just can not see the switch in the hole, use a paper clip that has been straightened out to gently push into the hole and feel for the button. then lightly press and hold following the procedure in the guide.
personally, I use the power interruption method (this might take a couple tried to get right but does work nicely) or the recovery ip (169.254.1.1) if a radio is being hard to get in to.
Really, the button is quite deep and small. You should try gently to fill the click with some straightened pin or small screwdriver. There are three options depending on press time. Less then 10 sec will reboot it, between 10 and 20 sec will erase config and more then 20 sec will initiate recovery procedure.