...
opkg update
opkg install atftp
cd /tmp
iptables -F
atftp -g -r openwrt-21.02.3-ipq40xx-generic-aruba_ap-303h-squashfs-sysupgrade.bin 192.168.1.225
ubidetach -p /dev/mtd16
ubiformat /dev/mtd16
sysupgrade -n /tmp/openwrt-21.02.3-ipq40xx-generic-aruba_ap-303h-squashfs-sysupgrade.bin
...