Hello, I am looking for guidance on how to flash firmware on my Netgear WNDR3400v2. I downloaded the appropriate file for it (I think) from here: https://openwrt.org/toh/netgear/wndr3400?s[]=wndr3400, but I am not sure how to flash the router. I tried 'router update' from the web interface, but that always complains that the file is incorrect or something. This will be a wired-only setup and only used to provide HDD storage to a single offline device via samba share. Thank you for your help!
EDIT: Here is the file that I downloaded and attempted to flash: http://downloads.openwrt.org/releases/18.06.1/targets/brcm47xx/mips74k/openwrt-18.06.1-brcm47xx-mips74k-netgear-wndr3400-v2-squashfs.chk