Django signals post_save method

I am creating API for online-store with django-rest-framework and can't get my post_save signal working for some reason. The idea is: I have a model Product and Category. Product model has a field named in_stock, which is True by default. As for model Category, it also has field in_stock, but here it is False. When the product instance created, I want assign Category field in_stock to True value using singals. Here is the code of models:

class Category(models.Model):
    category_name = models.CharField(max_length=255, verbose_name='Категория')
    description = models.TextField(verbose_name='Описание')
    in_stock = models.BooleanField(default=False, verbose_name='В наличии')
    slug = models.SlugField()
    image_url = models.ImageField(upload_to='categories/uploads/%Y/%m/%d/', null=True, blank=True,
                                  default='media/images.png', verbose_name='Изображение')

    parent = models.ForeignKey('self', on_delete=models.PROTECT, related_name='children', null=True, blank=True,
                               verbose_name='Родительская категория')

    def __str__(self):
        return f'{self.parent}, {self.category_name}'

class Product(models.Model):
    product_name = models.CharField(max_length=255, verbose_name='Товар')
    description = models.TextField(verbose_name='Описание')

    price = models.IntegerField(verbose_name='Цена')
    discount_percent = models.IntegerField(default=0, verbose_name='Процент скидки')

    in_stock = models.BooleanField(default=True, verbose_name='В наличии')
    amount = models.IntegerField(verbose_name='Количество')

    vendor = models.ForeignKey(Vendor, on_delete=models.CASCADE, verbose_name='Поставщик')
    category = models.ForeignKey(Category, on_delete=models.CASCADE, verbose_name='Категория')
    brand = models.ForeignKey(Brand, on_delete=models.CASCADE, verbose_name='Бренд')
    image = models.ForeignKey(ProductImage, on_delete=models.SET_DEFAULT, default=1,
                              verbose_name='Изображение')

Here is signals:

@receiver(post_save, sender=Product)
def update_category_is_active(sender, instance, created, **kwargs):
    print(instance)

    if created and instance.in_stock:
        instance.category.in_stock = True
        instance.category.save()

You can get it without signals by overriding save() method

class Product(models.Model):
    ...

    def save(self, *args, **kwargs):
        if self.pk is None:
            if self.in_stock:
                self.category.in_stock = True
                self.category.save()
        super().save(*args, **kwargs)

Did you add a ready function in the apps.py file? This is required as Django should know which files have additional code(like pre_save/ post_save) which should be executed when the server is running.
It will look like something like this:

from django.apps import AppConfig

class AppNameConfig(AppConfig):
    name = 'appname'

    def ready(self):
        import appname.signals