How to declare a list of lists in swift realm?

Hello! I´m working with Realm in Swift and have to map a json readed from a service with the this structure:

{
    "message": "",
    "status": "OK",
    "data": {
        
        "planesSesion": [
            {
                "nombre": "registro.turista.planDefecto",
                "id": 455412,
                "etapas": [],
            },
            {
                "nombre": "Camino Francés PErsoal",
                "id": 1247065,
                "etapas": [
                    [
                        {
                            "id": 1247066,
                            "class": "gal.smart.turista.util.sesion.TramoSesion",
                        },
                        {
                            "id": 1247067,
                            "class": "gal.smart.turista.util.sesion.TramoSesion",
                        },
                        {
                            "id": 1247068,
                            "class": "gal.smart.turista.util.sesion.TramoSesion",
                        },
                        {
                            "id": 1247069,
                            "class": "gal.smart.turista.util.sesion.TramoSesion",
                        }
                    ],
                    [
                        {
                            "id": 1247072,
                            "class": "gal.smart.turista.util.sesion.TramoSesion",
                        },
                        {
                            "id": 1247073,
                            "class": "gal.smart.turista.util.sesion.TramoSesion",
                        }
                    ],
                ]
            }
}

How can I declare “etapas” and his content to be mappable? Im getting error because is an array of arrays and trying to declare a list of an object who is a list too is always getting error “Expected to decode Dictionary<String, Any> but found an array instead.”

Best Regards!