To get FreeWRT into failsafe mode you need physical access to the device and the failsafe utility. The failsafe utility is built inside our ADK and is available in the directory bin/ after a successful build.
If you just want to compile the tool and not a complete firmware image, use following command:
[label=building the failsafe utility for the host system] $ make subdir=tools/failsafe install
The tool just opens a network socket and waits for a special UDP packet from the WLAN router. FreeWRT sends the UDP packet via the first recognized network interface (eth0).