Flash notmal firmware Install owut and luci-app-attendedsysupgrade Set required RootFS size:

$ uci set attendedsysupgrade.owut=owut
$ uci set attendedsysupgrade.owut.rootfs_size=512
$ uci commit

Set Advanced mode enabled in System -> Attended Sysupgrade -> Configuration Upgrade to the very same version of firmware using System -> Attended Sysupgrade -> Overview -> Search for firmware upgrade

That’s it.