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: Link>Double NAT and How NAT Works. 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.
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.