Thang Duong Bao

Bạn check certificate tại đây nhé: https://www.sslshopper.com/ssl-checker.html#hostname=https://6gem.net/

The hostname (6gem.net) is correctly listed in the certificate.
 	
The certificate is not trusted in all web browsers. 
You may need to install an Intermediate/chain certificate to link it to a trusted root certificate. Learn more about this error. 
You can fix this by following Comodo's Certificate Installation Instructions for your server platform (use these instructions for InstantSSL).
Pay attention to the parts about Intermediate certificates.