Why Ethernet connected devices go to 10.0.0.1/captiveportal.php with spinning wheel?

I thought cable connection to gateway should just work (it always worked with other internet providers in the past)

UPDATE: Then WiFi devices strarted doing the same thing - stuck in captriveportal on anything entered to the browser and the only thing that works is xfinity.com

UPDATE2: Gateway required activation - after 1 hour on the phone with support issue was resolved