thanks a lot for your help!
Well, I found a workaround that solved the problem. I have no clue why it worked but this is what I have done:
I uninstalled php (since I had installed it manually) and I tried to force Webinoly to install it by creating a site with Wordpress. When doing so Webinoly warns that PHP is not installed and asks if the user wants it to be installed. When I asked to do so previously, it would give me the error I mentioned of the GPG keys. So I tried with sudo stack -php-ver=7.4 and I got:
New PHP version: '7.4'
PHP is not already installed but this version will be installed next time!
then I wrote: sudo site my_domain_name.com -wp -ssl=on
And I got again:
+ PHP Not Found!
Do you want to install it now? [y/N]?
Error: retrieving gpg key timed out.
but then it kept installing and it actually reinstalled PHP, redis...
and now everything works just fine!!
I hope this might help
Thank you for your amazing support and software