Use subdomain for external cloud

Support
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1

Hi, I bought the domain example.com in netlify and would like to link the cloud I have set up (nc-1234.nextcloud-ionos.com) with the subdomain cloud.example.com.
For this, I have created a CNAME record that points to it.
However, when I call cloud.example.com 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 cloud.example.com in the address bar.
Is that even possible? What options do I have?
Thank you!

2

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