TP-Link WDR4900 firmware recovery by TFTP

Holding the WPS/Reset button pressed at least 15 seconds while powering up the router will make the bootloader of the device to try to connect to the IP 192.168.0.66 and request a file called "wdr4900v1_tp_recovery.bin" via TFTP. So there can be found a file ready for recovering if a firmware update fails. If the bootloader can&connect to TFTP server it will ignore the emergency and continue booting. In that case check your firewall, network settings, antivirus...