Unfortunately Optus have blocked the ability to change the DNS settings in their NBN modems
They are working on a fix for that. In the mean time you could apply the DNS changes to each device individually if needed.
DON'T hold your breath. Customers have been asking for the modems to be made usable since 2015. If Optus tell you they're working on it, it's a bald-faced lie.
Optus support, in general has been very poor. They have some great people but some are just outright argumentative. Recently, since they stopped answering their phones (Covid being a good excuse to explain poorer customer service) they have resorted to lying to customers via chat while remaining argumentative. End Rant!
In 2016 when I started with Optus I asked for access to the modem to change DNS addresses. "Sorry the computer says NO!"
I found a webpage that explained how to find out the admin password. I logged in and made the changes, at the same time changing the admin password.
Some time in between, Optus have upgraded the firmware and further locked down access to the modem, such that 1) they've reset the password to something they won't tell me and 2) the hack to get access to DNS entries no longer works. So I still had a modem with incorrect DNS entries and no ability to change them.
22 May 2020 I called Optus support. The password is on the back of the modem (is that underside of the modem?). No, that's the WiFi passphrase. It doesn't work.
They told me for over an hour that my modem LAN address was 192.168.0.1. I told them repeatedly that it had been 192.168.1.1 for over 3 years.
So we have an Optus support staffer, claiming to be a Level 2 tech who doesn't know anything about networking nor the modem.
So again, don't hold your breath for Optus to be helpful in allowing adults to manage their own network.
If Optus tell you they're working on it, it's a bald-faced lie.
In general I would agree with you. If general tech support told me that I would have dismissed it outright, but this confirmation came from Optus staff during a long running Sagemcom 5366TN discussion that included Optus contacting their network team, so I give it much more credibility.
I should have clarified in my original comment that the DNS fix was only discussed in the context of the 5366TN , and would not be for earlier models.
Is this the fix they've been promising since 2015?
No, Its the enabling of static DNS on the 5366 which they supposedly started working on in February, and went into testing at the start of November.
Optus has disabled admin access so that customers cannot make changes..
Correct. Is Optus the only one that does not give admin access?
A business decision that will not change unless customers demand it and vote with their feet if it's not delivered.
You advocate voting with your feet, but you still seem to be with Optus and have been since 2016.