If this modem has a built in router, it's best to bridge the modem. Having 2 routers on the same line can cause connection problems: To tell if the modem is bridged or not, look at the routers web page, Status/Device Info/Wan Section, if there is a 192.168.0.# address in the WAN IP address field, then the modem is not bridged. If the modem can't be bridged then see if the modem has a DMZ option and input the IP address the router gets from the modem and put that into the modems DMZ. Also check the routers DHCP IP address maybe conflicting with the ISP modems IP address 192.168.0.1. Check to see if this is the same on the ISP modem and if modem can't be bridged. If so, change the DIR router to 192.168.1.1 or .0.254.
Example of a D-Link router configured for PPPoE with ISP Modem bridged: PPPoE Configuration on Router
Check ISP MTU requirements, Cable is usually 1500, DSL is around 1492 down to 1472. Call the ISP and ask. Link>Checking MTU Values
For DSL/PPPoE connections on the router, ensure that "Always ON" option is enabled.