Supported devices
https://openwrt.org/toh/start

Update

opkg -verbosity=0 update
for PKG in $(opkg list-upgradable | cut -d" " -f1 | grep -v Multiple); do
    opkg upgrade ${PKG}
done

factory reset
https://openwrt.org/docs/guide-user/troubleshooting/failsafe_and_factory_reset

firstboot -y && reboot

FRITZ!Box 3490
https://boxmatrix.info/wiki/FRITZ!Box_3490
https://forum.openwrt.org/t/port-to-avm-fritz-box-3490/52692/77
http://www.aboehler.at/doku/doku.php/projects:fritz3490

FRITZ!Box 7360
https://boxmatrix.info/wiki/FRITZ!Box_Fon_WLAN_7360_v2
https://openwrt.org/toh/avm/avm_fritz_box_7360_v2

# update firmware
ssh 192.168.1.1
URL=http://downloads.openwrt.org/snapshots/targets/lantiq/xrx200/openwrt-lantiq-xrx200-avm_fritz7360-v2-squashfs-sysupgrade.bin
wget ${URL} -P /tmp
scp /tmp/openwrt-lantiq-xrx200-avm_fritz7360-v2-squashfs-sysupgrade.bin 192.168.1.1:/tmp/
ssh root@192.168.1.1
echo 3 > /proc/sys/vm/drop_caches
mtd -r write /tmp/openwrt-lantiq-xrx200-avm_fritz7360-v2-squashfs-sysupgrade.bin firmware

# from stock firmware
wget http://downloads.openwrt.org/snapshots/targets/lantiq/xrx200/openwrt-lantiq-xrx200-avm_fritz7360-v2-squashfs-sysupgrade.bin -P /tmp

Set your client IP to 192.168.178.2
Power on your your Fritzbox and connect to 192.168.178.1 via ftp in the first 5 seconds.

cd /tmp
ftp 192.168.178.1
login with adam2/adam2
type into the ftp prompt:
passive
binary
debug 1
quote MEDIA FLSH
put openwrt-lantiq-xrx200-avm_fritz7360-v2-squashfs-sysupgrade.bin mtd1