Open Traffic for WAN-Port (DHCP etc)

[xenon]

Hi,

first: Sorry for my bad english ;-) I try my best!

My Situation: Internet -> (WAN-Port->) ISP-Router (LAN-Port->) -> (WAN-Port->) ASUS RT-AC68U
that means: I have to use my ISP-Router and im going from its LAN-Port to the WAN-Port of my ASUS-Router.

My Problem:
I try to use the ISP-Router only for providing the Internet and i need the WIFI from it for 4 clients (not Radius Compatible). I use the Asus as DHCP-Server, Traffic Analyzer and as my normal WiFi (with RADIUS).
From my ISP comes a hybrid version of DSL and 4G. To manage, who becomes only DSL and who use the combination of 4G and DSL, the ISP-Router needs access to the LAN.
To use the WAN-Port of the Asus router, its necessary to be on different subnets of the ISP- and the Asus-Router.

Is there a way to disable this option or to get a workaround for this issue?

The goal would be, to get the DHCP-Informations over the WAN-Port (same subnet?) or the let the TrafficAnalyzer work on a LAN-Port.

I tried it now for 3 Days and didnt find any way to get it working.

For held i would be very grateful!

Greetings from Germany.