Not clear regarding, " Also you might check using directly the hostname as it might be possible that there is a bug when using variables."

I tried adding the variable for resolving the dns but still the ip address does not changes. Is there any other way? or is there any bug in my script.

resolver 8.8.8.8 valid=30s;
resolver_timeout 10s;
set $checkup "example.net";

location / {
rewrite ^/(.*) /$1 break;
proxy_pass https://$checkup:8080;
}

even adding the following codes instead of the above proxy pass doesnt work.

proxy_pass https://example.net:8080$request_uri;
proxy_pass https://example.net:8080;