You may have one or more variable on uboot misconfigured: http://www.denx.de/wiki/publish/DULG/to-delete/UBootEnvVariables.html
Anyway.. If the UI does not accept the image there is definitely something wrong. You also said you tried sysupgrade. Instead of using -F that is almost never going to result with a working router, you could try-v` to get a more descriptive error of what is going on.
If you access to console, it would be also interesting to see what the error is.