Its good that you got a workable solution.
If you are not using the wifi on the optus router, you might want to turn off its wifi (if you have not already done so) just so that it does not interfere with the D6400 's wifi.
hi guys just a quick update i finally had a small win with optus. i wasn't really that happy running 2 router so i made a conplaint to optus by email and after a week or so and phone call after phone c all i finally got to speak with someone in tech support that know what he was talking about and was able to make some kinda decision come to the party and sent me the new optus router segemcom f@st5366 and also gave me $99 credit on my next nbn bill. i got the router within 3 working days after set router up to my fetch mini oknyo amp and samsung smart tv i like to say that i am very happy with this new router fix my wfifi frop outs poor sound poor picture stop the feed back in my subwoofer speed is good so if you have the old router and having the same problem as i was for the last 12 months make a conplaint by email to optus and hit them up for this mew router segemcom f@st5366