For completeness, I got a big surprise when I received my monthly bill. Included was an increase of $120 over my recent bills. Phoned Optus and was explained that the said amount was for the new modem. Fortunately, things are hopefully resolved. The modem provided was not a 1-for-1 replacement. I was never advised that I would be charged for the replacement; if I had, I would have rejected it and looked elsewhere for an RSP (the 'faulty' modem was less than 12 months' old and, normally are under warranty). I did state here and elsewhere that I would return the replacement.
One piece of advice to all who are receiving poor service: first, threaten to go to the TIO - second, contact the TIO (I didn't) if Optus don't respond to your satisfaction.
PS: The TIO is unable to help if the issue is with NBNCo rather than Optus. Politicians - so sad.
WTF? Excuse my Chinese. That doesn't sound fair. What you are saying that, you reported a faulty device, an incorrect one send out to you, you offered to return the incorrect one and now you are being charged for someone else's mistake? Wow.
Update. The notebook's manufacturer (Asus) does not have a Windows 10 driver on its support web site. So, I contacted them by email and received a prompt response which directed me to a third-party site which did have the required driver. Installed it and the notebook now connects by wire to the 3864AC (so does my printer); Asus appears to have excellent support. I don't have to return the 3864OP that I was provided as a fix: have no idea what to do with it as it is useless as a spare for my wireless AC connections.
PS: I have no complains about the 3864AC's performance, particularly its wireless AC performance. I am on FTTN and won node lotto (typically 94/38 Mbps). The only negative is its lack of options for a knowledgeable user; I do understand why Optus has the constraints. I am happy to use it because I have retained my landline. If I cancelled my landline, I would definitely use a modem-router with better features and performance.
Now that my 3864AC is as good as gold, I have been 'exploring' its rather restricted GUI. There is a page entitled 'My Network'. It correctly shows the connected devices with the exception of the Fetch Mighty (not too concerned about that) and my Android devices (phone and tablet). The Android devices are shown as 'Cabled Devices (LAN)' and, of course, they are connected wirelessly - one AC and one N.
My dumb question is: why is it so? Probably just poor Optus firmware, but somebody may know. My thanks for an explanation.