The fact that you can access the TP-Link modem through you router makes me think the modem is assigning a local IP to your D-Link router. What IP do you use to access the modem? 192.168.100.1? This is the default IP of the modem based on the manual linked below.
Which is why I want to see the screenshot of the D-Link router Status page.
http://www.tp-link.com/res/down/doc/TC-7610(US_V1_UG.pdf