502 Bad Gateway Elixir/Phoenix - nginx

Question

I've set up a new Ubuntu 18.04 server and deployed my phoenix app but am getting a 502 error when trying to access it.
I don't yet have a domain name because I will be transferring one from another server, so just trying to connect with the IP address.
The Phoenix app is deployed and running, and I can ping it with edeliver.
Prod conf:
config :app, AppWeb.Endpoint,
load_from_system_env: false,
url: [host: "127.0.0.1", port: 4013],
cache_static_manifest: "priv/static/cache_manifest.json",
check_origin: true,
root: ".",
version: Mix.Project.config[:version]
config :logger, level: :info
config :phoenix, :serve_endpoints, true
import_config "prod.secret.exs"
Nginx conf:
server {
listen 80;
server_name _;
location / {
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
proxy_redirect off;
proxy_pass http://127.0.0.1:4013;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
}
}
Nginx Error log:
2020/05/14 22:28:23 [error] 22908#22908: *24 connect() failed (111: Connection refused) while connecting to upstream, client: ipaddress, server: _, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:4013/", host: "ipaddress"
Edit:
Last two entries of OTP logs confirming app is alive
===== ALIVE Fri May 15 07:33:19 UTC 2020
===== ALIVE Fri May 15 07:48:19 UTC 2020
Edit 2:
I have posted a Gist detailing all the steps I have taken going from a clean Ubuntu box to where I am now here: https://gist.github.com/phollyer/cb3428e6c23b11fadc5105cea1379a7c
Thanks

Answer 1

You have to add server: true to your configuration, like:
config :wtmitu, WtmituWeb.Endpoint,
server: true, # <-- this line
load_from_system_env: false,
...
You don't have to add it to the dev environment because mix phx.server is doing it for you.
The Doc

Answer 2

This has been resolved as follows:
There were two problems that required resolving.
Adding config :app, AppWeb.Endpoint, server: true to either prod.secret.exs or prod.exs was required.
I had a running process left over from mistakenly deploying staging to the same server, initially. I originally logged in to the server, and stopped staging with ./bin/app stop, maybe this left a process running, maybe somehow I started the process by mistake later on. Anyway, I used ps ux to list the running processes and found that one of the processes listed staging in its path, so I killed all running processes related to the deloyment, both staging and production, with kill -9 processId, re-deployed to production, and all is now fine.