I'm not sure I really understand what you're asking. so I'll go over a few possibilities.
First, you need to recognize that the D-Link 2750U is designed to be your primary "gateway" device. That means it's only meant to connect to the Internet via its DSL port and doesn't have a typical Ethernet-based WAN port available. If it is not being directly connected to a DSL line, it is only useful as a Wi-Fi AP and switch - Ethernet-to-Ethernet routing is not possible with this device.
On to the scenarios:
1. Your ISP provides you with a "dummy" DSL modem. (No routing/switching/WiFi functionality built in. Only communication ports available are 1 DSL and one LAN Ethernet.)
The D-Link 2750U cannot be connected to this device and serve any useful purpose. "Dummy" modems are designed to only connect to one device on the LAN, so you will need an Ethernet-to-Ethernet router (which the D-Link 2750U is not) if you want to share the connection around while using the ISP's modem as the gateway device. To use the D-Link 2750U here, you'll need to contact your ISP to obtain the necessary information and credentials for configuring it as your primary gateway. Then, you'd remove the ISP's modem from your network and put the D-Link 2750U in its place. This is the way it was meant to be used.
2. Your ISP provides you with a "home gateway" device. (This is a modem with routing & switching functionality built-in, and may also have Wi-Fi or other features added. These will usually have multiple LAN Ethernet ports.)
You can use the D-Link 2750U in this network, but it would be in a relatively limited capacity. Since you already have a device acting as a DHCP server and performing other routing functionality for your network, you would need to disable these functions on the D-Link 2750U before connecting it to the home gateway. Then, you'll connect a cable from one of the LAN ports on the ISP's gateway to a LAN port on the D-Link 2750U. From there, the D-Link 2750U would just be acting as a network switch and Wireless AP.