New Gateway

First. Is this a new service install / new move-in or an existing service ?  Is / are the light / lights on the modem lit steadily, or are they blinking (what color)? If lit steadily, did you get the device activated for service yet?

New move-in. White lights are blinking.

So, what typically happens is that the other end of the line has probably been disconnected from the Comcast feed. You likely need a tech out to reconnect your premises to the active Comcast feed at the main splitter or the tap on the pole for the building / premises. If it's a multiple dwelling you can try to get the owner / manager / maintenance person to do it. You won't be able to activate the service with the app if your line is not connected to the system. Good luck ! And please post back here with how things turn out.