It's tricky to work but my advice is to disconnect the Optus device and put it in a drawer for those times they ask you to connect it to diagnose why your Internet is down. You don't need it - with one exception which I'll get to in a minute.
Before our NBN was installed (HFC not FTTN) I tried in vain to get a clear answer to the problem of getting a new wi-fi modem when I already had a mature and extensive Airport wi-fi network. The problem being that I wasn't going to connect the 50+ devices I have all to the new wi-fi router just to use the Optus device which was in any case inferior to mine. In fact they gave me an 802.11N wi-fi when I already had the AC Airport Extreme so you can see how impressed I was.
The ISPs are giving or selling a wi-fi modem to every NBN user, which is great if you don't already have a wi-fi network, but a pain in the posterior if you have one already.
All I did the day we were connected to Optus NBN, and after the tech had departed having seen that I had the Internet working, was disconnect the Optus modem and plug the ethernet cable from my Airport router's WAN port into the black NBN box. The NBN box is in fact a modem - and the one that connects to the Internet. It's just like the old cable one you had to abandon when NBN was connected. When I connected the cable, I had my existing network running directly into the NBN box.
The model is Internet > NBN box/modem > Airport Extreme > Local Network
The exception mentioned above about the Optus modem is that you have to use it if you want to retain your fixed phone. The NBN requires your house phone to plug into the Phone socket of the ISP device (it's a green socket on my Optus device) and without it you don't get the phone to work.
However, as our household has a number of iPhones (in fact one each) we long ago abandoned our fixed phone. Perhaps you might have also? I'm told, but have never tested it, that if you connect the Optus device to your Airport router (in a LAN port) and the phone to the green socket that will be sufficient for the house phone to work.
None of this helps you get your current setup to work, but as I said you don't have to do it that way, despite what the ISPs tell you.