Hi @Happyo .
The VR1600 has to be connected to the NBN box and work in normal mode, not bridged, for the VOIP phone to work.
I don't know about using the AX in DMZ mode. It doesn't seem to be what you want. Unless you mean a real DMZ behind a firewall device?
The AX can be connected as an access point; AX WAN port to VR LAN port. I expect that DHCP and NAT functions of AX would be disabled. You would have to check if the VPN function can be enabled in this mode. You can disable the VR wifi and just use the AX wifi. You can check the relative wifi range of each router using a wifi device. You can still use the VR LAN ports.
An alternate config for the AX is to be in normal mode with DHCP and NAT disabled and connect AX LAN port to VR LAN port.
On the chance that the VPN server works in this mode, the mac address of the AX LAN port or WAN port needs a reserved ip address in VR. Set up the AX VPN server; the VPN subnet and netmask need a bit of thought. Set up port forwarding rule on the VR; external and internal port same as chosen in AX, internal ip address is the reserved address of AX, protocol is TCP and/or UDP. Set up DDNS on VR so you can use a hostname rather than ip address which changes.
You can experiment with the AX as the only router. Divert your land line to your mobile. Connect AX to NBN box.
Read this thread about configuring the AX.
