As @matinict mentioned in his comment, in Magento 2.3 you need to append
?templatehints=<Parameter Value> in your URL.
So your url will be look like as following:
and for any particular page URL will be like that
If you are using Magento 2.2 then on production mode, the template hint will not work.
Also, you need to check the setting at last level of scope mean store view scope.
Curiously. Now it works. Added a new block on the site and once again cleared the cache.