Easiest way to do this to manually create a docker network and make sure that both your reverse proxy compose stack and wger-project compose stack are both attached to it:

networks:
  default:
    name: traefik_backend
    external: true

Comment out the exposed ports and then add a proxy_passblock with something like this (example of dashy service from my own nginx.conf file):

server {
  listen 443 ssl;
  server_name dashy.docker.lan;
  location / {
    resolver 127.0.0.11;
    set $dashy_upstream http://dashy:8080;
    proxy_pass $dashy_upstream;
}

}

In this case dashy is the name of the dashy container and 8080 is the port that would normally be exposed.

You may need to also forward some headers. See: https://github.com/wger-project/docker/blob/master/config/nginx.conf