Self-signed certs are not trusted by Nginx Proxy Server.

So disable certificate verification.

Go to your proxy host custom locations and define / (root) location.

• Scheme should be https.
• Under advanced settings use this

location / {
    proxy_pass https://self-signed.example.com/:443;
    proxy_ssl_verify       off;
    proxy_ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
}

location / {
    proxy_pass https://123.123.123.123:9876;
    proxy_ssl_verify       off;
    proxy_ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
}

Add this in advanced tab.

location / {
    proxy_pass $forward_scheme://$server:$port;
    proxy_set_header Host test.com;
    proxy_ssl_name test.com;
    proxy_ssl_server_name on;
    proxy_redirect $forward_scheme://$server:$port /;
}