I think one of 2 things is happening. Either the modem is faulty or when you change the username and password there is a second step that we are missing to save the changes. I understand that you can click apply however there may be a second page where you have to save and reboot the modem to make the changes permanent.
Unfortunately Optus supplies very little information with their equipment so finding a screen shot to help you with this problem is proving difficult. There is also the possibility that I might be leading you up the garden path as this second page may not exist. Have a look around the administration section of your modem and see if you can find an option to save the changes and reboot.
If all else fails I would be contacting support again and telling them your modem is not retaining your authentication details.