Hi @Mise, thanks for posting.
Taking a look at your site, it doesn't look like it's using our hosting services and you appear to be using an SSL from Let's Encrypt.
From what I can see, 'www.slingbay.com' is not included in the SSL, so it's not secured.
You will need to work with your hosting provider or your SSL provider for assistance with this.
I am having the same issue. My site has everything purchased with godaddy. The domain name, the hosting the SSL everything purchased from godaddy. When I type https://hoteltravelagent.net it works fine. But when I type https://www.hoteltravelagent.net the browser says the site is not secured. Chrome works fine but other browsers report it non-secure site.