Just a precaution: I don't own this device.
- What firmware did you flash?
- Can you interrupt the boot process and end up in u-boot?
- And you could search in Belkin RT3200/Linksys E8450 WiFi AX discussion looking for terms like 'tftp' or 'recovery'
- on github linksys-e8450-openwrt-installer is maybe of help