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?

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.