Hard reset TP-LINK TL-WR842ND

Factory reset (Hard reset) Powered ON the router. The reset button is located on the back panel and it is labeled WPS/RESET. Use a thin object, like a paper pin, to press and hold the button for approximately 8 seconds. Release when the System LED starts flashing more quickly than before. Wait for the reboot process to finish – about 1 minute.
PS. The System LED is the first LED from the left on the front panel and looks like a small sun/gear wheel. 
Firmware Firmware link
Default username admin
Default password admin
Default IP address