Build an image using the instruction here
Build system usage with the following configuration:
Target AR71XX
Subtarget generic
Profile WZR-450HP2
Images ramdisk --→ gzip (will not work unless GZIP is selected, unselect squashfs not needed)
LUCI (luci is necessary to be able to connect to router from the browser)
After it is build find and rename openwrt-ar71xx-generic-wzr-450hp2-initramfs-uImage.bin (must be the initramfs or u didn t do the previous step correctly) to wgr450hp-ddwrt-uImage-initramfs-gzip.bin and copy it to a new folder
Download an tftp SERVER (example tftpd32) and make sure it can connect to the internet (firewall etc.)
Set your pc ip to 192.168.11.10 and gateway to 192.168.11.1 (i got this info from wireshark by listening to ethernet and searching for arp and the file name also from wireshark by listening to ethernet and searching for tftp)
open tftp server and set it to ip 192.168.11.10 and to the folder containing wgr450hp-ddwrt-uImage-initramfs-gzip.bin
unplug router
connect lan cable from router lan to pc (NOT wan)
press wps button and reset button at the same time and keep pressing
while pressing plug in router
continue pressing till red flashing led and then stop
if everything goes well the router will download bin file from the pc (check tftp server log)
WAIT
if led flashes red your .bin file didn t work check what u did wrong above
if led didn t flash red DONT UNPLUG ROUTER
Reset pc ip configuration done above, unplug and re-plug lan
connect to router via default ip
do a firmware upgrade