Nginx configuration for connecting to external mongodb from docker swarm

I have a docker swarm running 2 python flask services, each one running in a different region, say A, and B. Region A is also running Nginx, which is the main entrypoint to access the services. There is a MongoDB database running in region A, outside of the docker swarm. The python service running in region A can connect to the MongoDB running in region A, but service B cannot connect to the MongoDB from region B. My question is can I configure the Nginx to proxy service B to connect to the MongoDB?

The Nginx locations config for the services is:

location /a {
  proxy_pass https://a-service:5000/;  # running in region A
}

location /b {
  proxy_pass https://b-service:5001/;  # running in region B
}

Where a-service and b-service are the docker swarm container names.

I saw the posts regarding how to setup MongoDB behind Nginx, but my case is the reverse - to access external MongoDB from inside a docker swarm through an Nginx inside the swarm.

(How to setup MongoDB behind Nginx Reverse Proxy)

I understand that I need to have something like this in the nginx.conf:

stream {
    server {
        listen 27020;
        proxy_connect_timeout 5s;
        proxy_timeout 20s;
        proxy_pass    mongodb_host;
    }

    upstream mongodb_host{
        server https://5.150.225.25:27017;
    }
}

However the location of the mongodb host (https://5.150.225.25) is another vm outside of the docker swarm, not a local IP. This results in an error:

nginx: [emerg] invalid host in upstream "https://5.150.225.25:27017" in nginx.conf

Copyright Notice：Content Author:「J21042」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/60605888/nginx-configuration-for-connecting-to-external-mongodb-from-docker-swarm