@SuzieG when you say the optus modem is blocking the nighthawk, is it for a particular application or no internet at all? Please remind me of what type of internet service that you have, FTN, HFC etc? by the way what is the model number of your night hawk? does it have a wan port?
I have an HFC connection and tested with the Optus Sagem 3864ACv3 as the Modem and connecting an Asus wifi router from the Optus Lan port to the Asus Wan port, everything works, including fetchtv and optus phone. I say the nighthawk would work the same. If you already have the equipment, then I suggest you power on your nighthawk, only connecting the power. Do a factory reset on the night hawk. Turn it off after reseting. Next connect the Optus modem Lan port to the Nighhawk's Wan port. on a computer or the genie app, setup the nighhawk using the wizard. That should work.
I am now using an the Asus RT86 as my main router directly connected to the Nbn Aries box. Fetchtv work for me. I am on Nbn HFC, not sure if the same applys to other NBN type connections.