I was unable to get any of the previously discovered passwords (listed above by @abdullah1414) to work on my Telstra WiFi 4GX Advanced III (Netgear AC810s) running the latest firmware AC810S-1TLAUS_NTG9X40C_11.14.08.40_TELSTRA_05.02.
Instead, by extracting instructions from 4pda.ru I was able to change the ENTERCND password and then proceed to add the custom band configurations.
My particular use case has been local cells oversubscribed with visiting tourists (getting 0.2Kbps as a business customer), so I needed to force my modem to use alternate towers which I'm getting 30Mbps+ from.
Follow the instructions in this thread to connect to the device via USB and then telnet on port 5510.
$ telnet 192.168.1.1 5510
Connected to 192.168.1.1.
Escape character is '^]'.
Obtain a challenge:
Next, use a script contained in the 1_unlock.zip file from 4pda.ru to generate a response.
I have reproduced the script as a gist because it's a pretty wild ride learning how Russian numbers are written to get access to the file on 4pda.ru!
Configure Python libraries and generate the response:
python3 -m venv ./venv
./venv/bin/pip install pyserial
./venv/bin/python ./sierrakeygen.py -l DBA5D53217620C8C -d MDM9x40
Next, paste the response to the device:
I was then able to proceed with executing privileged commands, such as adding new bands:
AT!BAND=04,"LTE B1 2100",0,1
Hope this helps someone and saves them some time. So annoying that vendors don't keep these devices open by default.