M2 | Customer is not detecting by $customerSession->isLoggedIn()

I am trying to get loggedIn customer ID, I logged in and it is also showing in Online customer grid in the backend also can see on frontend. But if I check by

$  objectManager = \Magento\Framework\App\ObjectManager::getInstance(); $  customerSession = $  objectManager->get('Magento\Customer\Model\Session'); if($  customerSession->isLoggedIn()) {    echo "login"; } 

It gives out as not logged in. (using DI way also result is same)

Also

$  ID = var_dump($  this->_customerSession->isLoggedIn());     echo $  ID; 

It gives output as:

bool(false)

How can I get Logged In customer ID from session ?