Update OPENWRT 19.07 to ASUS RT-N16 and bypass ASUS' Firmware Check
ASUS now prevents consumers from updating to Open Source firmware and even downgrading to an older ASUS firmware. ASUS prevents consumers from extending the life of their older hardware which they purchased. By preventing Open Source firmware usage, ASUS puts the consumer at risk of Common Vulnerabilities & Exploits (CVE) which exist in every wifi router.
Is your router compatible with OpenWRT?
https://openwrt.org/toh/start
OR
Use the Open Source TFTP server
https://pjo2.github.io/tftpd64/