Hello,
https://www.victronenergy.com/live/ccgx:root_access#connecting_on_the_serial_console tells us not to connect the red wire.
That's correct as far as normal non-isolated TTL-to-USB cables are concerned, however I have a serial-to-USB adapter with built-in galvanic isolation (which is a super idea for this kind of work IMHO) – and that device does need its "red wire" plugged in, otherwise it won't work.
So, where is 3.3V on that connector?
Also you should probably mention that the USB-to-TTL cable in question should use, or be set to, 3.3V. There's loads of 5V cables out there. Don't use them.
