Intermediate/chained certificates are not supported here so if you have a custom domain you’ll always have this issue. Unfortunately it’s not something you can fix.
When you check your InfinityFree SSL certificate with online SSL checker tools, you might see a warning like this:
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.
This warning sounds scary, but it’s actually not a problem for your website visitors.
What this warning means for your site
You can safely ignore this warning. Your site will work perfectly fine with HTTPS for virtually all mode…