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Download the original firmware from Netgear
https://www.downloads.netgear.com/files/GDC/WNDR4300/WNDR4300-V1.0.2.104.zip -
Download openwrt 21.02.1 factory. Yes factory, not sysupgrade.
https://downloads.openwrt.org/releases/21.02.1/targets/ath79/nand/openwrt-21.02.1-ath79-nand-netgear_wndr4300-squashfs-factory.img -
Download, read instructions, and use nmrpflash to reinstall Netgear firmware. You need npcap too, and check that your firewall allows nmrpflash.
https://github.com/jclehner/nmrpflash -
Within netgear GUI, flash the openwrt factory image.
I manage a netgear 3700v4 which is a close model of the 4300. I had issues with it while upgrading from 19.07 to 21.02. It was solved by the method I described. Once 21.02 is flashed, you can safely use upgrading to the next releases.