I recommend that you phone contact your ISP service and request help from them as to maybe getting your current modem bridged.
I see that by the following PDF, that your modem may have bridging options:
http://www.buddybb.net/download/manual/billion_5400w.pdfDoes this look like your modems web page?
If so, they can bridge the modem, then you'll need to input your PPPoE user account and PW on the 600L router. If they can't help you do this, you could put the 600Ls WAN ip address it gets from the ISP modem inot the modems DMZ.
If any of this fails or can't be done, I highly recommend asking your ISP what model modem they support with out a built in router. You just need a modem only.
If that should not work, you could try this:
Turning a router into an AP.Other suggestions maybe instead of using a DIR-600L, get a D-Link DAP model wireless AP and install on the back end of the ISP modem to provide better WiFi and turn OFF the ISP Modems WIFI.