Any chance of an upset became even harder after 28 minutes when Fosu’s wayward shot cannoned off the thigh of Johnson to give the home side the lead at a packed Sixfields.
Kettering keeper Dan Jezeph produced two great saves from Liam McCarron to keep the non-league side in the contest, and they gradually took over in the second half and pinned the Cobblers back.
With Ranger starting, two more former Premier League strikers, Gary Hooper and Marvin Sordell, came on for the Poppies, but it was teenage substitute Miller who changed the course of the game.
Ben Hart swung in a wicked cross from the right and the local boy timed his run to perfection to guide a picture-book header into the top corner.
Northampton tried to respond but with Johnson marshalling the defence the away side held firm, with Aaron Powell’s sliding block to deny Akin Odimayo being especially heroic.
Just two minutes into the extra 30 minutes, Miller took a hand again, drifting in an excellent corner which was met by Ranger with a thumping finish.