The site loads properly on serverIP:5870 and if I change proxy_pass http://127.0.0.1:5870; to proxy_pass http://listmonk.mydomain.com:5870; then it will load on listmonk.mydomain.com:5870. But it gives the 502 error when I visit the site without the port.
The connection for this site is not secure
listmonk.mydomain.com sent an invalid response.
[Try running Windows Network Diagnostics](javascript:diagnoseErrors()).
ERR_SSL_PROTOCOL_ERROR
502 usually means it's having a hard time hitting the service on the back end. Are you able to access listmonk directly without the reverse proxy? What is the URL I. The browser when you do that?
Is listmonk on the same server as nginx? Is nginx running in a container? If nginx is running in a container, 127.0.0.1 would try to load from inside the nginx container which isn't where listmonk is. Use the LAN IP address of the docker host listmonk is on instead of 127.0.0.1.
Are you able to access listmonk directly without the reverse proxy? What is the URL I. The browser when you do that?
Besides the info I put in the OP, I'm not sure what else you're asking.
LAN IP address of the docker host
No idea what this is, so I looked it up https://www.howtogeek.com/devops/how-to-get-a-docker-containers-ip-address-from-the-host/ and ran docker ps then put the container ID at the end of this docker inspect -f '{{range.NetworkSettings.Networks}}{{.IPAddress}}{{end}}' docker-container-ID. It output an IP address which I used for proxy_pass http://docker-IP:5870;, restarted nginx, and nothing seems to have changed.
Not the docker container IP. The IP of the machine you are running docker itself on. Nginx is running in a container, it's not allowed to talk to the other docker container IP directly and it has a separate network stack from the listmonk container, so 127.0.0.1 only goes back to nginx.
Say your machine that you are running docker on is 192.168.0.67, the listmonk docker container is 172.16.0.89, and nginx container has an IP of 172.16.0.34.
Your nginx config would need the proxy to point to 192.168.0.67:5870.
Then make sure you don't have ufw or some similar firewall blocking the connection from nginx to listmonk.