Nginx Reverse-Proxy Only Works On Internal Network

I wanted to accomplish the following goal and get it working on an internal network, however, this doesn't work externally.

Goal: Deploying nginx to reverse proxy n servers with different private ip addresses, and only open ports 80/443 on the router and forward it to the Nginx server in the backend.

Ngix version 1.18.0 on Ubuntu 22.04 LTS (Jammy Jellyfish)

I have a domain name which points to my router's public ip, and I have a nginx reverse proxy config which only works good when I'm connected to home wifi (internal network), but backend servers won't be accessible if I try my LTE on my phone, for example.

Here is my reverse-proxy.conf:

    server {
        server_name my.domainname.com;
        access_log /var/log/nginx/reverse-access.log;
        error_log /var/log/nginx/reverse-error.log;
        
        listen 443 ssl;
        listen [::]:443 ssl;
        ssl_certificate /etc/ssl/certs/localhost.crt;
        ssl_certificate_key /etc/ssl/private/localhost.key;
        ssl_protocols TLSv1.2 TLSv1.1 TLSv1;


        location /guac/ {
        proxy_pass http://privateip1:port/guacamole/;
        proxy_buffering off;
        proxy_http_version 1.1;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection $http_connection;
        proxy_cookie_path /guacamole/ /guac/;
        access_log off;
        }

        location / {
        proxy_buffering off;
        proxy_http_version 1.1;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection $http_connection;
        proxy_pass http://privateip2:port;
        
        }
        

        location /api {
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_set_header X-Forwarded-Host $host;
        proxy_set_header X-Forwarded-Server $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_pass http://privateip:port;
        }


    }

    server {
        listen 80;
        listen [::]:80;
        server_name my.domainname.com;
        return 301 https://$server_name$request_uri;
    }

For more information, I saw it on the phone that browser was trying to access private ip address 192.x.x.x, then I tried to make another port forwarding rule on the router and point it to that server, but still got the same unreachable error on my phone after replacing the private ip with the public ip of the router with that newly opened port. I also allowed the firewall rule on that server to accept tcp/udp on its port and make sure nothing is on the way, but still no luck.

So, my question is, is it possible to accomplish this goal with nginx reverse proxy?

Please let me know if you need more info.

Thanks,