Upload OpenWRT Firmware for Asus RT-N66U via tftp / curl

1. Set your computer to a static IP (e.g. 192.168.1.123), subnet 255.255.255.0, default gateway 192.168.1.1
2. Put router into recovery mode by holding down the reset button while powering up the router, wait until the power led is blinking slowly, then release the reset button.
3. upload .trx file (brcm47xx/mips74k/) with

   curl -T openwrt-18.06.4-brcm47xx-mips74k-asus-rt-n66u-squashfs.trx tftp://192.168.1.1

4. When complete, wait until the router reboots itself, which can take at least 10 minutes.
5. Wait until the router reboots itself, then access the OpenWRT GUI (Luci) at 192.168.1.1 to set a password.