I noticed mine has some zen autofill entries even after a factory reset with custom firmware.
Then it is a Zen branded device and not pure vanilla ZyXEL.
I'm not sure but I think mine might be already unbranded since I'm able to flash custom firmware directly.
If you see fields pre-populated with Zen's configuration then the device is not pure ZyXEL. Have you cleared ROM-D for your device? If not, then that will be where Zen's configuration is stored.
I just discovered my supervisor password now thanks to this thread https://forum.kitz.co.uk/index.php/topic,19186.120.html
I assume you have logged in as Supervisor and turned off the original ISP's TR069 etc settings?