The following should be a complete sequence to enter failsafe mode. I struggled to find a complete version after bricking my router.
Set IP address for ethernet card to static address to communicate with router.
sudo ifconfig <ethernet adapter> 192.168.1.155 netmask 255.255.255.0
Direct cable to LAN1 in router
Open listener for UDP packet to enter failsafe mode
sudo tcpdump -Ani <ethernet adapter> port 4919 and udp
Power on router.
During startup, the router will broadcast a UDP packet requesting a button press to enter failsafe mode. When this message is displayed in the terminal listener, press the WPS button on the router to enter failsafe mode.
The power button will begin a fast blinking mode.
ssh into the router as root.
ssh root@192.168.1.1
Enter commands to reset router to factory install.
firstboot && reboot