If I understand correctly, your ZS box only has one interface, but you want to use it as a router between your ISP router and your home network? Or are you just offloading services to the ZS box so your ISP router is more efficient?
For the ZS box as the router, it is almost impossible with one interface and stuff WILL be exposed.
For the ZS box to offload the services from the ISP router, it depends on the services. DHCP, DNS, no problem. Turn them off on the ISP router and set them up on the ZS box instead. Port forwarding and traffic shaping/monitoring, no way in hell.