It's saying "mismatch" between your client device and the FGT. It's NOT saying the FGT doesn't support the encryption level your device is requiring. Go to CLI under "config system global" then "get | grep admin-https-ssl". You should get like below:

  fg50e-utm (global) # get | grep admin-https-ssl   admin-https-ssl-versions: tlsv1-1 tlsv1-2 tlsv1-3 Then you need to make sure that the browser's supported TLS versions would overlap with them. Each browser would have different way to do the settings, which you need to figure out for your browser.

This issue usually involves a problem with your web browser or your site’s SSL certificate. The browser’s telling you that because it’s trying to tell you there’s a problem with the certificate the website is using for HTTPS, so “this site can't provide a secure connection”. In all cases the end-to-end encryption is still going to work just because HTTPS can’t function without it. There is no definite guide for managing this error.

Two possible options to get rid of this issue

1. Use Self Sign certificate
2. Remove domain security policy:

Steps for Chrome :

• Go to : chrome://net-internals/#hsts
• Query HSTS/PKP domain for localhost
• Use Delete domain security policies option to delete configuration for localhost

This error is because of the following problems:

• Invalid SSL or SSL is untrusted (self-signed)
• SSL Not installed properly
• Old Technology or SSL/TLS version for encryption