The problem here is that you access the cameras though ports 80/443, and ZS is alrready listening to those ports.
Two steps that may help you:
1 – Disable ZS admin interface from outsiders and make it accessible just from internal (LAN) clients (This is also a good security measure)–> this can be done under Setup / HTTPS Tab / “Allow access only from” and select just your LAN interface (be careful to select the LAN and not the WAN interface as this can prevent you from accessing the admin pages from the LAN if you pick the wrong one).
2 – Since you’ve probably NATed your WAN interface, you can then use Setup / Network / Virtual Servers to map port 80 to one of the cameras OR select several different ports, one for each camera and translate each to port 80 of each camera’s LAN IP (you may also need to add firewall rules to allow such accesses in case you change the default FORWARD policy to DROP or REJECT). Note that if you choose ports different from 80 and 443 for each camera, step one is unecessary, but will still be a good security precaution…