Go to the outer status page. The router should display whether it has an internet connection to your provider. If no, then your router/modem has no credentials or another issue preventing access.
If it shows as working, then you can narrow it down to incorrect DNS and IP routing. Perhaps dynamic IP allocation is set to off or another configuration error or bug, in which case you might need to reset all the router settings. Then, is it only broken for a single end device?