It is possible the bindings on the server were not updated. Check this by remoting into the server. Go to the IIS Manager, click on the site then go to the Bindings in the Action menu

Then click on the site Host name and Edit

Then look at the dropdown for the SSL certificate. Choose the current certificate listed in Rock. You can find that under Installed Plugins/ACME Certificate. Look at the date and time.
Your browser may cache the old certificate even after caching Rock. Open the site incognito window to see if it is secured.