I have a custom DB9 cable that uses pins 2, 3 & 5. Can I use this for serial communication from the PC to the DCN-(W)CCU?
No. You will need a full 9-pin null cable, since the (W)CCU communication on multiple pins.
Details on the RS232 connector can be found in the installation and User Instructions manual since version 3.1x for example found in the DCN-NG software ISO under docs.