To solve the reboot > setup loop after you've done the setup reboot the router from the interface, wait 4 minutes then access again, this will act as a hard save and then if you reboot again (presumably you're doing so via the physical button or pulling power) then it shouldn't go back to the setup. Try the upgrade immediately after a reboot/factory reset i.e. reboot/reset and try to access as soon as you're able to to initiate the upgrade.