Connect your computer to the WLAN router via direct or crossed network cable. Use the failsafe port (in most cases one of the LAN ports), see the device specific page for the exact network port.
Configure your network interface to the IP address 192.168.1.2 with network mask 255.255.255.0. Now start the failsafe utility on your computer.
$ ./failsafe
After that power on your WLAN router and wait for the following message in your failsafe application running on your computer:
Press reset now to enter Failsafe!
As soon as this message is displayed you should push the reset button of your WLAN router. You have 2 seconds time to push the button. If you successfully enabled the failsafe mode, following message will be displayed:
Entering Failsafe!
Now you should be able to login to your WLAN router via a telnet application. Just use:
$ telnet 192.168.1.1