Hello,
LXC container based on Ubuntu 18.04 should have only a static IPv6 address, as configued in /etc/pve/nodes/pve/lxc/100.conf:
net0: name=eth0,bridge=vmbr0,firewall=1,gw=192.168.9.100,gw6=2001:1570:218:9::1,hwaddr=56:0A:77:22:13:0F,ip=192.168.9.203/24,ip6=2001:1570:218:9::203/64,type=veth...