The reason why it had been disconnecting every 4 or 5 minutes is because two different modems from two different locations were trying to authenticate at the same time, even though both modems are set to dynamic and there were no broadband username and password set to the modem.

I borrowed the iiNet supplied modem Technicolor TG789 and gave to one of the company's owner as it's being connected to NBN at the same time. I didn't want to use theTechnicolor as port 443 would not open up for ActiveSync and OWA, client has Windows SBS Server 2008, hence I used the existing Draytek Vigor2760 and no issue with port 443. Setup for the Vigor2760 for NBN FTTN can be found here.

iiNet modem's firmware are lock with some settings which they couldn't find themselves.