Use subdomain for external cloud

Hi, I bought the domain in netlify and would like to link the cloud I have set up ( with the subdomain
For this, I have created a CNAME record that points to it.
However, when I call I get the error ERR_SSL_PROTOCOL_ERROR.
This is probably caused by the Lets Encrypt certificate. I have therefore initiated the update of the certificate manually, but unfortunately without success.

My questions are: How can I use the subdomain for my cloud? I don’t want a simple redirect to this page, but always work with in the address bar.
Is that even possible? What options do I have?
Thank you!

If you point the CNAME externally, it’s up to the other server to provision a SSL certificate for that domain. Netlify only provisions SSL certificate for the domains hosted on Netlify.

OR you can use Netlify Rewrites: Rewrites and proxies | Netlify Docs