RC-AC87U and the nbn

I have a FTTN NBN connection and Telstras TG799vac, aside from a few issues with the modem itself I want to move back to my much more advanced Asus RC-AC87U, mainly for QoS etc.

My issue is this:

How do I connect the two and maintain all functionality of NBN. (VoIP and the VDSL)

To maintain Qos functionality the asus router has to be in Wireless router mode

I've tried a number of different combinations to no avail.

Bridging the TG799vac, The asus router has a couple different ways of connecting to the internet within the Wireless router mode.

1. PPPoE, L2TP, PPTP
2. Cable modem, Dynamic IP, DHCP, Static IP

My most recent attempt got me an error message from my Asus router "Your ISP's DHCP does not function properly"

This was when using Wireless router mode -> Automatic IP WAN connection type.

What settings should I have on the modem to have another router like the RC-AC87U in use?

Any assistance would be great