Ditto what Monnat said, as it would take a debug interpretation by Tech Support to determine the exact issue you're seeing.
Both Eth ports on the front of the ZD are one logical interface, switched together, for a PC or additional AP/Printer/etc.
But if you have a switch into one of the ZD ports now, connecting your additional PC/Printer/APs on the switch should
all be able to communicate with your ZD. Even though you are using static IPs, do you have have a true router to act as
your little network's default-gateway? All all devices using the same default-gateway IP address?