Your phone is assigned an IP address by your router. If your phone does not have an Ip address, it will be unable to communicate over the internet.
You can check the Ip address assigned on your phone. In most cases, the IP address will be something like 192.168.x.x or 10.x.x.x. If your phone does not have an Ip address, try rebooting the router. If this does not help, check the physical wirring and contact your network support. If you find that you regularly need to reboot your router to get an Ip address for your phones, it is probably time to replace your router
On a Yealink phone, press the "OK" button in the center of the arrow keys. The screen will then show the status of your phone including the IP.
Press the "Menu" key and then Select Status