I see. So you already have your Core claimed and named, hence you don’t need to go through all that (including rainbows) again.
Are you sure, you’re seeing cyan breathing and not white (if you look closely at the LED no red dot allowed - otherwise it’s white - don’t get blinded by it ;-))
There might be some misunderstanding. You can use
Serial in your firmware to “listen” for input, but listening mode (blinking blue) is something else.
Once you enter you WiFi credentials via serial your Core should go from blinking blue (listening) to white, to green blinking, to cyan flashing, to cyan breathing.
Does this happen?
Have you by any chance tried
WiFi.off() in one of your tests?
Have you tried flashing new firmware via USB?
Have you confirmed that your company network has port 5683 open?