The cert may live on the edge device so it could be that the HTTPS connection is between the browser and the edge device. The edge device may then be connecting to IIS over HTTP (instead of HTTPS), if this is the case it may be that there's no cert in the settings for your bindings (wwwroot should still be 'https://...' in this scenario).
One thing to check: if your IIS server is in house and the browser you're trying to connect with is on the internal network but the certificate is installed on the edge device then does the moodle.domain.com domain name resolve to the internal IIS server address or the external edge device address. You sometimes have a scenario where a site's domain name resolves to a public/Internet IP address for external users, but resolves to an internal/intranet IP address for internal users and this may need to go to the edge device instead, or alternatively needs to resolve directly to the IIS server in which case IIS may need the certificate installing and a port 443 binding.
Also purging the server-side cache after the config.php change may help (you may need to work out where your php.exe and purge_caches.php files are).