Thanks @bobmc but actually it's working now. All I needed to do was set the correct bridge interface.
I changed it from vmbr0 to vmbr1 and things started working.
root@pve ~ # grep ^net0 /etc/pve/nodes/pve/qemu-server/401.conf
net0: virtio=1A:68:AE:F6:D7:E0,bridge=vmbr1,firewall=1
I created...