Just want to add for others. If its in a non booting state sometimes uboot on the AR300M will reboot and try to TFTP an image. If you do not have a serial connection, fire up Wireshark and look at the network traffic of the device. You may see it requesting an image file. You can unbrick it by setting up a TFTP server and serving the image file it is requesting. It will flash it and reboot and you’ll be back in business.
This is only if you do not have the WebUI of Uboot or a netconsole or any other nicer method.