Redis – celery .delay freezes for this task but runs for others

I am trying to send notifications using celery.

@shared_task(name='send_notifis')
def send_notifs(device_ids, title, message):
    from pills_reminder.models import UserNotifications, UserDevice
    devices = UserDevice.objects.filter(id__in=device_ids)
    print(devices)
    device_tokens = []
    for device in devices:
        UserNotifications.objects.create(
            uid=device.device_id,
            title=title,
            message=message,
        )
        print(UserNotifications)
        device_tokens.append(device.registration_token)

    if len(device_tokens) > 1:
        device_tokens = ["".join(token.split()) for token in device_tokens]
        response = push_service.notify_multiple_devices(registration_ids=device_tokens,
                                             message_title=title,
                                             message_body=message)
    elif len(device_tokens) == 1:
        registration_id = "".join(device_tokens[0].split())
        response = push_service.notify_single_device(registration_id=registration_id,
                                          message_title=title,
                                          message_body=message)
    else:
        pass
    print(response)
    return True

this works without .delay() and when running using
python manage.py shell

>>> send_notifs.delay(devices, title='title', message='message')
<AsyncResult: f54188f8-cec6-42dd-a840-b097abffd7f4>

but it freezes when i call using Django Model post_save signal.

@receiver(post_save, sender=Notification)
def Notification_post_save_handler(sender, instance, **kwargs):
    print('hello from post_save signal')
    devices = instance.get_devices()
    # send_notifs(devices)
    if len(devices)>0:
        send_notifs.delay(devices,
                    title=instance.title,
                    message=instance.message)

This above code freezes execution, but without .delay. it works fine.

UPADATE:1

the above task with .delay is running from python manage.py shell not from runserver . so the problem is with celery and Django settings. Hence i dig deep and found out

while running from shell i get,

>>> add.app.conf #(add is a async task here)
{'broker_url': 'redis://localhost:6379/1'}, ...

but running from runserver gives:

`{'broker_url': None}`

Now i am looking for how to set the settings properly ? I am using django-configurations with celery.py as

from __future__ import absolute_import, unicode_literals

import os

from celery import Celery

# set the default Django settings module for the 'celery' program.
os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'core.settings')
os.environ.setdefault('DJANGO_CONFIGURATION', 'Development')

import configurations
configurations.setup()

app = Celery('core')
app.config_from_object('django.conf:settings', namespace='CELERY')
app.autodiscover_tasks()


@app.task(bind=True)
def debug_task(self):
    print('Request: {0!r}'.format(self.request))

Any help is appreciated. Thank you for your time and patience.