If you find unable change the "Redirect to HTTPS turned off" on Domain function. You can go to Wordpress Tools, click to Issues and the message will be prompt as following:

Permanent SEO-safe 301 redirect from HTTP to HTTPS is turned off. Visitors can still make unsafe connections to your website. Turn on the redirect to HTTPS in the Hosting Settings to address this issue. Turn on redirect to HTTPS. 

1. Click to the link to Turn on redirect to HTTPS.
2. The link will redirect you to Domain, now you can switch it to On now.

If still not working, you can logon to Wordpress and install a plugin named Really Simple SSL and enable redirect from HTTP to HTTPS. 

If still not ok, modify the file .htaccess located on public_html. Add the following at the end of the file

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

If you are facing a mixed content issues, add this code to your website’s .htaccess file:

<ifModule mod_headers.c>
Header always set Content-Security-Policy "upgrade-insecure-requests;"
</IfModule>