Hi Bongo, welcome to the forums.
I wasn't aware the Telnet password could be changed for an HG612 modem, but I could be wrong. If you are using a different modem then you need to select that from the list available.
Anyway the answer is quite easy*, navigate to the Scripts folder, and then the Editor folder.
In there you will find Modems.txt, open it and change the 0 after HG612 to a 1. This will make the password field in the GUI editable.
*I say easy, but a have sneaky suspicion that it still won't work especially as you've hinted that you have changed it in the INI file, although the above will stop the GUI changing it back. You see I wrote the GUI but I didn't write the code that actually gets the stats and it may be hard coded in that code. Unfortunately we have been unable to contact Baldeagle1 who wrote the code for well over a year. All I can suggest is to try the above and see how you get on.