No need to do all of this.
- Just use a default OpenWrt config with the normal LAN setup.
- Then see: Guide to set up DMZ via LUCI
You simply make a Port Forward rule to assign open all ports to the device. This would be your very last firewall rule. Simple.
EDIT: I realized that the OP provided that link already.
@L-Warlok ...please explain why you're making VLANs, etc., when the solution was to simply make a firewall rule???