As report the official guide it works.. (on the first lxc) .. but which is the correct /dev/net dev/net for others tun devices?
nano /etc/pve/lxc/123.conf
Add the following lines at the end (if you're using PVE < 7.0, change `cgroup2` with `cgroup`)
lxc.cgroup2.devices.allow: c 10:200 rwm...