Hard reset TP-LINK TL-WDR3500

Factory reset (Hard reset) Power ON the router. Locate the WPS/Reset button on the right side of the rear panel. Press and hold the WPS/Reset button for approximately 8 seconds (make sure you hold it for more than 5 seconds). Release when the the SYS LED becomes quick-flash from slow-flash and wait while the router reboots to its factory default settings.  PS. The SYS LED is the second led on the front panel – the one that looks like a small sun.
Firmware Firmware link
Default username admin
Default password admin
Default IP address