A device for mounting a ring gear of an epicyclic gear train, the ring gear including a front ring gear and a rear ring gear, the device including: a plurality of longitudinal guides; a front mounting forming a portion of circle, including: at least two holes for passing the guides; a plurality of first system for supporting and rotating the front ring gear relative to the axis thereof, movable between a first position in which the front ring gear is supported and can rotate relative to the axis thereof and a second position in which the front ring gear is not supported; a rear mounting forming a portion of circle, including: at least two holes for passing the guides; and a plurality of second system for supporting and rotating the rear ring gear relative to the axis thereof, movable between a third position in which the rear ring gear is supported and can rotate relative to the axis thereof and a fourth position in which the rear ring gear is not supported.