aptitude install tftp-hpa
Now you need to configure your network to connect to the router and create a static arp entry for it. Directly connect the router to your computer by ethernet cable, but leave the router powered off. You will need the MAC address for the router, you can get it from the SSID on the label, just insert colons like: 001D12345678 = 00:1D:12:34:56:78. You will need to do this as root.
ifconfig eth0 192.168.11.2
ifconfig eth0 netmask 255.255.255.0
arp -s 192.168.11.1 00:1D:12:34:56:78
eth0with the ethernet interface you are using, if necessary (it is probably
eth0), and use the MAC address from your router, as described. But the IP address should be as shown, not based on your usual network configuration. Now change to the directory where you downloaded the firmware image and start up your tftp client and prepare to send the image to the router (don’t power it on yet). Once you start the tftp client, you will be typing at it’s command prompt.
tftp> rexmt 1
tftp> timeout 60
tftp> put openwrt-15.05-ar71xx-generic-wzr-hp-g300nh-squashfs-tftp.bin
(Replace the name of the image, if you are using a different one.) As soon as you hit enter on that last command, plug in the router. The tftp client will keep retrying until the bootloader on the router is ready to receive the image, then you should see the upload begin to progress. Wait a few minutes and the router should reboot. Now you can turn your normal networking back on.
If you installed OpenWRT, the default IP for the router will now be 192.168.1.1, not 192.168.11.1. If your image included LuCI, like the one I used, you can now point your web browser to that IP to access the web interface. Now you have not only unbricked your router, but you are free to enjoy all of the awesomeness of OpenWRT!
Most of the instructions here are based on this post, many thanks to the author!