Bit confused about your modem question. The modem is the first connection to your ISP, so you should already have one, that D-Link you have now is the modem but it's also your router. So you don't want to connect a modem to the D-Link, you want to replace the D-Link with a regular modem.
If you want to upgrade the router, check with the company which DSL modems will work with their system, and get just a modem, then you can hook that up to your router and be set. Or you can check the options in the D-Link and turn off DHCP and the router features on it, and use it just as a modem. Should have a method to do that.