Have you done it exactly as it's described below?
Quote from https://openwrt.org/toh/asus/rt-n13u#tftp_firmware_install
Steps:
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Set your computer to a static IP 192.168.1.15 (Other IPs will NOT work).
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Plug your computer into any LAN port
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Turn the device off and wait a few seconds. Hold the Restore button in with a pen and insert power. Sometimes Restore button does not work, but WPS button does, use that.
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Wait until the WPS light blinks slowly and then let go of the Reset button. Your router is now in recovery mode and has the IP 192.168.1.1. Note it will not respond to pings.
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Send a firmware image using command-line tftp; eg. from Linux:
$ tftp
tftp> binary
tftp> connect 192.168.1.1
tftp> put openwrt-ramips-rt305x-rt-n13u-squashfs-sysupgrade.bin
Sent 3407876 bytes in 1.2 seconds
tftp> quit