There is a password for the ACS URL and it's not the same for everyone.
I got my hands on two unmodified modems and they each have a password. Unfortunately it seems to be tied to the modem serial number in some way as the password was different on each one.
There could be some kind of pattern to the password, and the username definitely contains the serial number of the unit.
So I guess you need to record that data on your modem prior to modifying any tr69 settings or flashing firmware.
The password is blanked out in the GUI screens even when using a root password (find it somewhere else). You must login with telnet and issue the command "dumpcfg" then search for the XML section for ManagementServer to see the real settings on your modem.
(note, no real passwords in the above, it's just an example of the XML you will find in your own router config files)
Using the username and password from another modem might be a possibility to get the firmware back but I would not want to leave it enabled with another modems serial number. You could end up with Telus pushing commands to the wrong or both modems.
Going to the ACS url in a browser with valid username and password results in the result of:
CPE Servlet responding
Active session count = #