LAN-port instead of switch before WAN port (AC68u, Merlin 380_63.2)

[legrand]

First router serves some public computers in a DMZ. One of the cables runs to another apartment to an AC68u (WAN-port). After the AC68u follows the internal network.

-> (wan)router(lan) ---> switch1_DMZ ----------------> switch2_DMZ -> (wan)AC68u(lan) ---> internal

To add a shared host belonging to DMZ but physically next to the AC86u I simply added a switch (switch2_DMZ) before the AC86u and attached it to this switch instead of the AC68u lan port.
Works perfect, but I wonder if I could spare the switch2 and use one of the AC86 LAN-ports for that purpose: Have one of the lan ports acting exactly as the ports on switch2_DMZ
Is there an easy/safe way to realize that?

 

[L&LD]

I don't think you can assign a port a DMZ, but you can assign an IP on the 'AC68U. Don't know if it will work or not, but easy to try.

 

[legrand]

I don't think you can assign a port a DMZ, but you can assign an IP on the 'AC68U

from the AC68u's view the switch2 is simply the EXTERNAL (WAN) side. I mentioned the DMZ because I wanted to show the whole picture.
My question basically is: can I treat a lan port on the AC68u as a 'pass-thru' of the WAN port, i.e. have it behave the same way as a port in switch2 (in order to omit switch2 to save some electricity)?