Subdomain A record does not point

I have setup the primary’s domain name A and CNAME records, and they work properly.
I tried to point the A record of a subdomain to another IP but it does not work. The DNS checkup shows that the subdomain points to Cloudflare ip address

My desired structure is as follows
stergioubooks. com A record --> IP address 1 (WORKS)
www CNAME --> (WORKS)

erp. stergioubooks. com A record --> IP address 2 (Does not work, points to
www. erp CNAME --> erp. stergioubooks. com (does not work, points to

I have read relevant articles herein and I saw that solutions. However, in my case nothing works.
Any ideas?

Thank you

In the meantime, in order for my site to work, I use another domain to point the IP address 2 from a hosting provider (not cloudflare NS)

PS 2
Disabling the another provider did not solve the problem

PS 3
the main domain (and the subdomain, of course) points to Cloudflare name servers.

Though should work, the reason why it does not is that you dont have a valid certificate for that hostname on your server, as the error message suggests.

Thank you, so much, Sandro.
I will follow your advice and post the results.
Many thanks again.

Yes, it works.
Million thanks