The miner still doesn't allow me to upgrade the software, still showing the "413 Request Entity Too Large" problem. My S9 has an SD slot though, so i might need to try boot from there/flash from there?
Have you tried to flash it first with upgrade fix? You can found it from here
s9_fix_upgrade.tar.gz if not then flash this first before you flash it with the default firmware.
If the result is the same then the only solution you can do is SD card recovery but I don't hear someone success on flashing antminer s9 with SD card because Bitmain doesn't release an SD card firmware or a guide from their support page.
There 2 video how do they flash the S9 and make a bootable SD card recovery.
-
https://www.youtube.com/watch?v=dzArOe8KuHI&feature=youtu.be-
https://www.youtube.com/watch?v=VT0GBoh_xTIAnd another solution that I think may fix this issue is this one
Rescuing a bricked T9 (and S9?) Control Board - HOWTO. This is your final solution if BitMiner247 solution doesn't work this is the final solution to fix this issue.