Nginx Reverse Proxy - proxy_pass using "FQDNs" - nginx

Question

We have been trying for days (we tested hundreds of setups) to make a Nginx Reverse Proxy successfully reverse proxy a web application that needs FQDNs (this is mandatory for this web application to work).
Using the configuration below for the Nginx Reverse Proxy together with a local DNS service (see resolver attribute) that knows the FQDN we can successfully make these http calls...
server {
access_log /var/log/nginx/apps.mbr.domain.abc-access.log;
error_log /var/log/nginx/apps.mbr.domain.abc-error.log;
server_name *.apps.mbr.domain.abc;
location / {
proxy_pass https://$host$request_uri;
resolver 127.0.0.1:53;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_ssl_server_name on;
}
listen 443;
ssl_certificate /etc/letsencrypt/live/apps.mbr.domain.abc/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/apps.mbr.domain.abc/privkey.pem;
include /etc/letsencrypt/options-ssl-nginx.conf;
ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem;
}
... , however if I change the proxy_pass attribute to using an IP as shown here...
server {
access_log /var/log/nginx/apps.mbr.domain.abc-access.log;
error_log /var/log/nginx/apps.mbr.domain.abc-error.log;
server_name *.apps.mbr.domain.abc;
location / {
proxy_pass https://10.2.0.18:443;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_ssl_server_name on;
}
listen 443;
ssl_certificate /etc/letsencrypt/live/apps.mbr.domain.abc/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/apps.mbr.domain.abc/privkey.pem;
include /etc/letsencrypt/options-ssl-nginx.conf;
ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem;
}
... the web application reports not knowing the URL (error). In other words, clearly there is some parameter/data (we don't know what it is) that is added by the DNS service to the http call.
QUESTION: What is the local DNS service provided parameter/data that Nginx Reverse Proxy is not providing?
NOTE: We are asking this because we believe this is something that can be provided by the Nginx Reverse Proxy itself so that we will not need to use the local DNS service.
Thanks! =D