How can I filter documents from a collection by a string type field named "fecha_proximo_mantenimiento" (next_date_maintenance)?

Hopefully I understood your question correctly.

The query below will filter documents that have fecha_proximo_mantenimiento with value older than 30 days.

[  {    "$addFields": {      "fecha_proximo_mantenimiento_timestamp": {        "$subtract": [          "$$NOW",          {            "$toDate": "$fecha_proximo_mantenimiento"          }        ]      }    }  },  {    "$match": {      "fecha_proximo_mantenimiento_timestamp": {        "$gte": 1000 * 60 * 60 * 24 * 30      }    }  }]

First stage $addFields adds new field (I called same as original with appended _timestamp and it subtracts field value (which is also converted to Date, since your schema defined that as String) from Date.

Second stage $match checks that subtracted result (in miliseconds) is greater than 30 days (you can also replace with 2592000000 if it's easier for you).

const hasta = moment(new Date()).add(30, 'days').format('YYYY-MM-DD');const desde = moment(new Date()).format('YYYY-MM-DD');const contrato= 0;Dispositivos.find(  contrato,  fecha_proximo_mantenimiento: {     $exists: true   }},  {    _id: 0,    'informacion_equipo.ususario_red': 1,    fecha_proximo_mantenimiento: 1  })  .where('fecha_proximo_mantenimiento')  .gte(desde)  .lte(hasta)  .exec(function (err, result) {    if (err) {      console.log(err)    } else {      for (var x in result) {        console.log('numero de documento: ', x, result[x])      }    }  });

It works too with find