Your router may have been hacked to direct DNS queries to a malicious server.
Follow the manufacturer's instructions to reset the router to the default state. Usually that involves inserting the end of a straightened paper clip or a similar tool into a pinhole somewhere in the back of the device, and pressing a switch inside for about 15 seconds. The pinhole may be marked "RESET."
Repeat the initial setup process. Make sure the router does not allow remote setup from the Internet (WAN port), if it has that feature—most do. The DNS servers should be set automatically by your ISP. If you still have trouble with those servers selected, contact your ISP.
Check the router manufacturer's website for a firmware update.
If you have a wireless network, it must be secured with WPA 2 encryption. The passwords for the network and the router must each be a string of at least 10 random upper- and lower-case letters and digits, and they should be different. Any password that you can remember is weak.