Issue Preferred domain does not redirect www.example.com to example.com

[gsk]

I have a certificate that includes only the domain and not the subdomains.
Therefore under settings / preferred domain , I have selected example.com in order to redirect all the traffic from www.example.com to example.com, thus my secured domain.

The strange thing is that if I select www.example.com, it works. It redirects traffic from example.com to www.example.com but the opposite fails.

Unfortunately this does not work.

I do not work to use another certificate for the subdomain www.example.com.
I simply want to redirect subdomain www to my domain.

I have tried setting directives through apache settings but plesk seems to ignore them.

I use external DNS and I have a CNAME record that points to domain, thus www.example.com and example.com point to the same ip.

Thanks in advance for your help.