kevpatts,
Did you read the info on: https://openwrt.org/toh/linksys/e8450
And on: https://github.com/dangowrt/owrt-ubi-installer ?
The second link is how to make a ubi version for your router.
It tells also the following:
Upstream firmware version 1.1.x rejects the installer image. The recommended work-around is to downgrade to version 1.0 before running the installer.
Are you running version 1.1? Then you first have to downgrade your firmware version. also described on second link. The 'normally' build .bin files can be run on the device, but only ones. After a reboot stock-firmware will be active again.
Furthermore only snapshot version are working for this device, so I presume there is a lot more to find on the forum, like Wifi: Belkin RT3200/Linksys E8450 WiFi AX discussion for this device (2704 replies?)
DG.