I am new to this Instagram API, and I read their doc about endpoints, this is the endpoint that I am using:
/v1/tags/{tag-name}/media/recent?access_token=ACCESS-
Your client is in Sandbox Mode and can only search for tags of photos posted by invited users. You have to login into https://www.instagram.com/developer, edit your client and click on the "GO LIVE" button.
If the "GO LIVE" button is disabled, you have get your app reviewed by Instagram first: Click on the Permissions tab and submit for review. (Company Name, Contact Email and Privacy Policy URL are required to start a submission.) Once approved, u will be able to click Go Live.