There is no place I would stop on the RER Line before Gare du Nord so basically, my suggestions are all in Paris. You could go into Paris and exit at Gare du Nord and have a meal at Terminus Nord. I last ate there in May 2018 and it was good -- not amazing but good. What about L'Etoile du Nord (par Thierry Marx)? It is located right in Gare du Nord. I have not been yet but I like the concept. Another to consider Chez Casimir, which offers hearty seasonal cooking and a lot of old Paris charm. This one is the furthest away from the station (5-minute walk away). Food options at the airport are not great but if you go into Paris, you do want to make sure that you do not miss the last train to CDG. Is the Brasserie Flo (Terminal 2F at CDG) closed or not an option?