D-Link DWP - 157 not working with OS X EL Capitan ?

If it is a wireless issue, see my FAQ*:

http://www.macmaps.com/WIFI1048.html

If it is a wired issue, make sure you are plugged in the right ports. WAN only goes to modems that have a LAN port. LAN goes to the computer. And make sure the cable itself is rated for the speed you hope to be getting from the router.

Test with only one device hooked up to the router, to make sure there isn't any interference from other devices.

Firmware issues with routers are very common if the router is older than 5 years. Make sure the most recent firmware is installed.

Make sure ethernet cables have their phone cable like tab pop back up inside the port for them. If it is down, it means it is not firmly seated. And if it is not there, use a different cable.

Try using Open Source DNS on both router's built-in DNS settings and Apple menu -> System Preferences -> Network Settings.

Use the Network Connection doctor on the Network Settings if it isn't green.

* Links to my pages may give me compensation.

Have you tried restarting/resetting the router?