By connecting the D-Link's LAN ports to the ethernet modem and the PC, it is functioning as a basic network switch. You will get online with one PC, but it isn't functioning as a router, which would support the connection of multiple computers to the internet.
At the moment, the D-link is passing the single IP address allocated by your ISP directly to your PC. What should happen is that the *router* gets that single IP address as its WAN address and, in turn, allocates LAN IP addresses, usually in a 192.168.x.x subnet, to the computers connected to it.
There shouldn't be any "configuration" of the WAN port, but you need to go into the routers Setup (type its IP address into a web browser) and configure it to connect to your ethernet modem, which is just a basic connection, possibly described as "direct" or "DHCP" or something similar. I assume that, at the moment, the Zoom modem connects automatically to the ISP and that you aren't having to establish the connection from the PC? If you are having to run a PPPoA connection session from the PC, then you may not be able to use the router correctly as a router, ADSL normally requires a combined Router & ADSL Modem.