NBN Fast 5355 and Asus DSL-AC68U with VoIP Phone

We recently received our NBN connection kit in the mail however I'm a bit hesitant in connecting it as I have a small home network with specific needs.

My current setup is an Asus DSL-AC68U (most up to date firmware) on an ADSL connection. It handles everything in my home network; Wireless, DHCP, VPN, DDNS, Firewall, Parental Control, guest networks, I have a lot of reserved IP addresses for computers, print server, several computers which have shared drives, and a couple of VLANS (provided by HP Procurve 24 port switch).  Home phone line plugs into the ADSL filter box.

The Telstra Network Gateway is the F@st 5355; and unfortunately I have had lots of problems (probably the same with other people) with them in the past setting them up for friends/family. There are a few problems that I would need to overcome in order to keep my current home network setup with NBN.

With my current home network and set IP Addresses I really need to use the 192.168.1.x IP Range, however the F@st 5355 doesnt like to have its default IP Range changed from 10.0.0.x. It might work for about 30 minutes but then goes belly up and only way to fix it is to factory rest the modem.

If I were to put the F@st 5355 into Bridge mode we would lose the phone line. It wouldn't be such a big problem normally however we are in an area with terrible mobile phone reception. We usually have to go out side to get any kind of signal. So putting the F@st 5355 into bridge mode isn't really an option.

In the NBN Connection kit there is a white box that looks similar to an ADSL filter; one end plugs into the wall outlet and there are two cables coming off of that labeled DSL and PSTN. I have been brainstorming ways of connecting to the NBN whilst still using my existing network with settings and also having the VOIP phone working. Would something like the below picture work? 

I would also appreciate any input from others in similar situations.