lxc launch ubuntu:20.04 tftp

lxc config device add tftp eth0 nic nictype=physical parent=enp0s25

lxc file push Downloads/RT-N66U_3.0.0.4_382_52272-g73d3ea2.trx tftp/tmp/
lxc shell tftp

apt update 
apt install -y tftp

ip l set dev eth0 up
ip a add 192.168.1.111/24 dev eth0


# ping 192.168.1.1

# tftp 
tftp> connect
(to) 192.168.1.1

put RT-N66U_3.0.0.4_382_52272-g73d3ea2.trx

Links
https://chrishardie.com/2013/02/asus-router-firmware-windows-mac-linux/