Are you with Xfinity? Do they have a quick start guide? There’s usually some kind of registration process to be completed.After that, you usually need to set up your preferred SSID and password for WiFi, if that’s the goal. Have you tried connecting a laptop/PC directly to the modem/router via ethernet and checked that level of access?

I am connected through ethernet cable.  The modem is not connecting to wifi .. The Comcast part is completed !!! 

I want to know how to change the name of the modem also ..

T25 is the cable modem and there is no Wi-Fi inbuilt in it. There is no option to change the modem’s name since there is no Wi-Fi in it.

If you prefer for the Wi-Fi access, I suggest you to connect router with this modem.