I don’t have one of the ipq40xx-based, GL.iNet routers in my “stable”, unfortunately.
sysupgrade -v might provide more insight. Also, as
sysupgrade is a shell script, so
sh -x /sbin/sysupgrade ... is another tool (check the path, that’s from memory).
In my experience, errors like that are often from missing metadata in the image’s configuration, either in whole or, for example, SUPPORTED_DEVICES.
It is also possible that you have QSDK-based firmware running. Though based on an old version of OpenWrt, it has many differences that QCA have included, most of which are proprietary and source is not publicly available. I haven’t looked into the original GL.iNet firmware for the S1300. If the banner (or other files in
/etc/) indicates “Barrier Breaker” or “Chaos Calmer”, you are likely running a QSDK-based build. There is nothing “wrong” with QSDK; in many cases it can outperform open-source firmware.
U-Boot is another option for flashing, perhaps a bit safer than blindly trying to “force”