Are buses and or Metro available?
Gare du Nord is just a short walk away to get to the RER B to De Gaulle. You could also take the Metro No 4 or No 5 line one stop to Gare du Nord.
It's probably easier to stay above ground and do the five minute walk between Gare de l'Est and Gare du Nord, where you get thr RER B, than to go down the stairs under ground and walk the tunnels to the platform. If you look on a map you'll see they are next to each other.