Hi OpenWrt-folks,
I recently bought a used Asus RT-AC56U router. I made a reset and logged into the default router UI. I configured the router and it worked fine.
I noticed that a new firmware version was available. I chose to let the router UI take care of the upgrade. The upgrade failed. The router became unresponsive - no access to UI, no internet. Dang.
With the router in recovery mode, first do a basic connectivity test with ping 192.168.1.1.
If you get no reply, check if your ARP table now has an entry for 192.168.1.1. If there is none, the router is
unreachable over the network and flashing attempts will also fail.
If connectivity is established, watch the tftp transfer with the tftp client's verbose mode or tcpdump/wireshark.
Try a different network cable, network interface, PC, tftp client software or operating system and repeat the previous tests.
Maybe a serial console would show what is happening on the target.
Disclaimer: I don't have this device.
Openwrt 15.05 is unmaintained. The current stable release for the RT-AC56U is 18.06.1.
Either one should be OK for unbricking, though.
A hardware defect would certainly be a good reason to dump the device.
Firmware problems should be fixable depending on your available time, since the bootloader seems to be intact.