Serial Console would be your friend here. I don’t have a B1300 yet, but the pins are present and documented on all of the GL-Inet stuff I’ve ever used so B1300 will probably be the same.
U-Boot console can be a dangerous place if you’re not careful, so don’t try running any commands there unless you are certain; I would suggest simply connecting the console, put the device into the UBoot WebUI mode, and trying to do your upload. The console will show you UBoot’s output and why it is not accepting the file/etc. You can then troubleshoot from there.