OK Guys,
I have been switched to NBN now and the first thing I noticed that T-Gateway is a single band WiFi (2.4 GHz) only, which is bad for me as this band is fully occupied by my neibourhood. In order to keep my old TP-Link (a very good router with dual band) as wireless access point I have managed to hook it up to T-Gateway without any problems. Now, I have my internet modem and phone line connected to T-Gateway and local WiFi handled by my old router at 5GHz band.
Very simple very good configuration :-)