The answer
Create an A record: Type A, Name api, Value (your Elastic IP). Then add an Nginx server block with server_name api.yourdomain.com and the appropriate proxy_pass or root. Finally, get SSL with sudo certbot --nginx -d api.yourdomain.com.
Configuring DNS Records for AWS EC2 A Records, CNAME, and Subdomains
How do I set up subdomains like api.yourdomain.com?
More FAQs in Configuring DNS Records for AWS EC2 A Records, CNAME, and Subdomains
First allocate an Elastic IP and associate it with your instance. Then create an A record in your DNS provider: Type A, Name @ (for root) or api (for subdomain), Value (your Elastic IP). Verify with dig yourdomain.com A +short.
The default public IP changes when you stop and start the EC2 instance. An Elastic IP is a static IP that persists across stop/start cycles. Without it, your DNS A record would point to the wrong IP after a restart.
Create a hosted zone for your domain, update nameservers at your registrar, then create records: A record for root (empty name, Elastic IP value), A record for subdomains (api, Elastic IP), and CNAME for www (yourdomain.com).
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