Check ISP MTU requirements, Cable is usually 1500, DSL is around 1492 down to 1472. Call the ISP and ask.
For DSL/PPPoE connections on the router, ensure that "Always ON" option is enabled.
If the ISP 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>http://www.practicallynetworked.com/networking/fixing_double_nat.htm and http://cognitiveanomalies.com/cisco-nat-how-nat-works/. 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.