Ok, I found the problem, when I created the subdomain, it was under Document Root /public_html/static instead of just /public_html.
However, only my product images are served via the subdomain and my GTmetrix still have a "F" for most of my pages.
I also noticed that the subdomain that I have created on my Host now has "http://static.mydomain.com/%{ENV:REWRITEBASE}img/c/$1$2.jpg" as a value for Redirection, probably created with the updated .htaccess file. My site have a lot of .png images also, and these are not served via the subdomain. How can I have them also served via the subdomain?
Do I have to modify the .htaccess file, probably at the section under "# Images" to also include .png images to be served via the subdomain?
Any recommendation would be appreciated.
Cheers
Dan