Recover your possible brick via Failsafe mode

The following HowTo is written for the MR3020, but usually the most OpenWRT based routers work in that way with the failsafe mode.

Connect the MR3020 to your computer via Ethernet.
Set up your of the computer to 192.168.1.2 with subnet mask '255.255.255.0' (This step differs from OS to OS of your computer.)
Start to ping 192.168.1.1 (this is the failsave IP) infinitely
Power the MR3020 on and when the WPS Button starts to blink, push it (it will blink faster)
You see the ping getting response
telnet 192.168.1.1
run mount_root
Now you can edit files