http://track-chinapost.com/startairmail.php shows 83621667255 result as "shipping notice received from seller(WISH邮平台)" . This means your seller applied a tracking number from China post, but never actually shipped anything. You should be cautious of fraud.
I suggest you to push seller ship the goods immediately. Tell him if you can not track real shipment info in next 3 days, you will open dispute against fraud.