Android Studio app link assistant digital asset li

Question

For my android app project , i wanted to use app deep linking, using app link assistant. Following steps I have done:

1. Added url intent filters.
2. Logic to handle intent
3. Associate website - created DAL file using debug certificate.
4. Save the file as assetlinks.json
5. Save the file under my server https://sub.subdomain.example.com/.well-known/assetlinks.json
6. The file is accessible via browser.
7. But when tested inside the android studios app link assistant link and verify, it give error "The website hosting the Digital Asset Links file must support HTTPS with a valid certificate. Please ensure the DAL file is accessible at https://sub.sudomain.example.com/.well-known/assetlinks.json".

I am using below configuration on server:

1. Ubuntu 16.04
2. PHP 7
3. SSL certificate generate via letsencrypt.org
4. fullchain.pem and key.pem used in SSL configuration
5. nginx/1.10.0

6. Android Studio 2.3

   Can someone guide what could be possible issues? Why the android studio app assistant not verifying the file.