I believe Telstra does do the same thing, not sure. The password does in fact get changed by optus for various versions of the modem updates or rollouts, so its a moving target. I visited this post back in 2017 and got some advice then and have never looked back.
So you're stuck with the modem but you can attach a router.I had a great Netgear router, 6300 I think and applied the instructions from the following post. I have since replaced my router with a Nighthawk R7000 which gives me great cocerage in a large house, and a very large capacity for MAC connections which I now need with so many power points converted to google home/ewelink. The R7000 setup was fantastic - it dd all the bridging form me in the wizard setup. Honestly, buying a good router/modem and bridging is the easiest option.
Secret is to use your router as the DNS server, and simply turn off DNS server in the Optus modem. The Optus modem is likely to be 184.108.40.206, so the router will be 220.127.116.11, and all your connections will be on THAT dns. Here's the very good post that I found and still refer to from time to time! Good luck!
We've never enabled bridge modem and from all the discussions I've had with network and product, there's never been talks around enabling this feature.
I understand that @Dan_C , and don't dispute that. But there are easy ways to use a secondary modem router to achieve what people are looking for without impacting the Optus provided modem and in particular telephone system. Netgear up to date modem/routers setup wizard achieve this in ROUTER mode (not BRIDGE though available) with no user input which has been fantastic. I'm sure the others like d-Link do too. Its worth it.
If using a router, basically Optus Modem wifi is turned off, and the router only links to the Optus modem. WiFi devices link to the router which has internet access in Optus modem.
At any one time I have 26 devices and counting attached to my R7000, the older one had a limit of 16. I now have a limit of 64 (32 on each band 2.4/5ghz). Latest modems achieve 96 devices in tri-band routers Optus Sagem has a limit of 32 devices (16 on each band). The coverage and speed I achieve with the add on router far exceeds the Optus modem, which is older technology. So in the home, if you have a lot of older equipment/devices that only access 2.4ghz, your 16 devices slots could fill up quickly.