Summarize this article:

If you can connect a phone, tablet, or computer to the network and access the webpage that you're having trouble with on your primary device, the issue is definitively with the device and not the router.  Not being able to connect with the second device doesn't necessarily mean that the router is the problem. This is one of the quickest ways to test your DNS connections. Download a different free browser such as Firefox or Chrome and attempt to connect to the internet; if the problems persist, you can rule out browser problems as the reason for your DNS server not responding. If the issues are resolved, uninstalling and then reinstalling your old browser will likely fix the problem. This will clear your router's cache and thus may resolve DNS errors. To do so:  Unplug your modem's power cable as well as your router's power cable. Allow both your modem and your router to sit for at least 30 seconds. Reconnect your modem and wait for it to come back online. Reconnect your router to your modem and wait for it to come back online. If you're already using Ethernet, skip this step.  If you can connect to a web page while using Ethernet, the problem may stem from your router. You'll most likely need to reset it. If you can't connect to a web page while connected via Ethernet, your DNS settings may be the problem.
Try connecting with a different device. Try a different browser. Power cycle your modem and router. Connect your computer to your router via Ethernet.