Hi AsusMY,

Would like to seek your advice.

I bought Asus DSL-AC68U AC1900 router yesterday to replace my unifi dlink dir-615 stock router.

Somehow i just notice that there is no dedicated WAN port since the router is 2 in 1 WAN/LAN

My question is which router port should i plug the LAN (1-4) if im using port 1 modem?

Encounter error "pls check your dsl/wan"

I cant get it to work since this evening. Really need your advice thanks.

u bought wrong wan bro..
u bought dsl mean for steamyx not for unifi
tht why no WAN port
only DSL port

You bought the wrong router.
Understand this:
RT-AC68U - Wireless router only - This is the one you should've bought.
DSL-AC68U - ADSL Modem + Wireless Router - ADSL is for Streamyx only, you cannot use this for UniFi.

From your picture, that white modem is the BTU (broadband termination unit), you can't remove/replace this.
You can only remove/replace the D-Link DIR-615.

Look at your BTU again, notice the black ethernet cable connected to Port 1?
The other end of the cable has to be connected to the WAN port of your router.
Unfortunately, the DSL-AC68U does NOT have WAN port, it only has a DSL port.