Along the version upgrade, you are also switching target, ar71xx to ath79, and that causes the error. Use sysupgrade image, but you need to sysupgrade without settings and using the force flag once the the image check fails in LuCI. ( sysupgrade -F -n in console)
The way Linux describes hardware has changed during the years, and the old devices in ar71xx target have been modernized into ath79 target.
And your 19.07 sysupgrade attempts failed likely because you tried the factory image (that is meant for flashing from the OEM firmware)
You might also check your device's openwrt wiki page if there is something special regarding sysupgrades from ancient versions.
After the **sysupgrade -F -n *.img command, the machine becomes bricked and restarts infinitely! Fortunately, the machine comes with an immortal boot, switch to tftp mode and use ...netgear_wndr4300-squashfs-factory.img to update successfully!
Checking the machine version is the premise of the upgrade, the firmware version is correct, this can't be wrong! The problem is that there is a problem with the checking mechanism of sysupgrade. The configuration description of version 20 and versions before 19 has changed a lot, so it cannot be passed. Finally, the update is successful through the tftp method of the machine.