Some tips for setting up:
1: Download and install the dual wan firmware for the ASUS N55U here:
https://www.asuswebstorage.com/navigate/s/907241C47DA14B46BC9928E7681A3534Y
Note that this is beta version 3.0.0.5.364 for the dual wan, and the latest official version 3.0.0.4.364 -- you MUST install ths firmware if you want to run the asus as a router only and have a modem behind it.
2: Connect your TG589 to your pc (make sure lan adaptor is set to obtain automatic ip for this whole procedure) -- connect your phone line to the TG589 also!
Browse to 192.168.1.254 and access the modem, click "Technicolor Gateway" on the left hand side, and then click on "Setup my Technicolor Gateway". Follow the wizard that pops up, make sure Bridge is the selected service type and that ipv4 dhcp is enabled.

3. Disconnect your PC from the TG589 and connect it to back to your asus with its newly installed firmware. Connect a ethernet cable from LAN port 1 on the TG589 to LAN port 1 on the ASUS.
4. Browse to your asus webpage, login, goto WAN on the left and the Dual Wan at the top. Set as follows:
Enable: ON
Primary WAN: DSL
Secondary WAN: LAN -- Lan Port 1
Multi Wan: Failover
Enable the routing Rules: No
Click Apply.

5. After it applies, click "Internet Connection" at the top (left of dual wan), set as follows:
Wan type: Lan
Connection Type: PPPOE
Enable WAN: Yes
Enable NAT: Yes
Enable Upnp: Yes
Get the WAN IP automatically: Yes
Connect to DHCP server: Yes
Username: user@xtrabb.co.nz
Password: password
MTU: 1492 (Telecom tells you to set 1500, that won't work, and it won't make a difference)
Service Name: TELECOM (For some reason you MUST do this, it would not work until I did this)
Leave everything else as default.
Click apply.

Attempt to browse to any webpage, it should spit out about and ip conflict (Technicolor is already using 192.168.1.X) and attempt to automatically do it for you (it should change your asus lan ip to 192.168.2.1). If it doesn't, you can do this manually in the asus routers page by going to LAN >> LAN IP.
Done! You should be online without double routing your connection. If not, try a reboot on both devices and try apply the settings on the asus again and wait a minute. Great connection speed and no nat issues with xbox live (for the gaming fanatics out there).