sub domains and dns challenge

[maxmonz69]

Hello,
I admit I'm quite a newbie regarding SSL certificates but this is my situation.
I would like to have a certificate for my domain (i.e mydomain.ddnsfree.com) and using it for all subdomains (i.e. foo.mydomain.ddnsfree.com, bar.ddnsfree.com).
I created a reverse proxy for mydomain.ddnsfree.com and I generated a certificate using DNS challenge (with cloudflare). Everything is fine.
Then i created the reverse proxy for foo.mydomain.ddnsfree.com and I assigned the same certificate.
When I navigate on foo.mydomain.ddnsfree.com the connection results not secure like there isn't a certificate.
What is wrong?