Do you already have a wired "old fashion" doorbell? Typically they're already hooked up to a transformer (ours is mounted to a rafter in the attic) as doorbells need the line voltage dropped from 110V (US) to 24V or lower, depending on the doorbell.
@Agatha I don't from what I see unless they took it out and hid it under the siding. I haven't checked the attic though that a good idea. I'm going to check the attic to see.
Look for a long thin 2 wire cord. Often red and white, but not always. Start from where your existing doorbell is and follow it back.
@Agatha confirmed not up there. returning smart door bell plan to by a battery operated one.