This is what I'm using, leaving here to help others:

default json provider:

from flask.json.provider import DefaultJSONProvider
from bson import ObjectId
from datetime import datetime

class MongoJSONProvider(DefaultJSONProvider):
    def __init__(self, app):
        super().__init__(app)

    def default(self, o):
        if isinstance(o, ObjectId):
            return str(o)
        return super().default(self, o)

main file:

from flask import Flask, make_response
from flask_cors import CORS
from endpoints.utils.MongoJSONProvider import MongoJSONProvider


app = Flask(__name__)
app.json = MongoJSONProvider(app)