Nginx Load Balancing – Upstream SSL Name Configuration

I have the following Nginx configuration to balance the load among different nodes. However, when I try to redirect the traffic I obtain 502 Bad gateway.

By reading the error log I found out that the problem is related to the fact that my Nginx load balancer is trying to verify the validity of the X509 certificate NOT for the various nodes (backend1.example.com,backend2.example.com), but for the name of the upstream backend.example.com (without the number), leading to the error shown below.

How can I tell nginx to use the hostname of the forwarded node, instead of the one of the upstream?

ERROR LOG:

upstream SSL certificate does not match "backend.example.com" while SSL handshaking to upstream...

CONFIGURATION:

upstream backend.example.com {
   least_conn;
   server backend1.example.com:443
   server backend2.example.com:443
}
server {

        listen [::]:443 ssl ipv6only=on;
        listen 443 ssl;
        server_name example.com;

        location / {
                proxy_pass https://backend.example.com;

                proxy_ssl_trusted_certificate /etc/ssl/certs/ca-certificates.crt;
                proxy_ssl_session_reuse on;
                proxy_ssl_verify       on;
                proxy_ssl_verify_depth 2;
                proxy_set_header Host $host;
        }
    ssl_certificate /etc/letsencrypt/.../fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/.../privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}