Is it posible to use jwilder/nginx-proxy in port 80 for docker apps and local apache in port 81 for undockerized apps?

Question

I've already deployed on my server the jwilder/nginx-proxy and some apps running with docker, and it's working with no problems.

Now, I want to deploy some apps that don't have docker using the apache web server installed in my ubuntu server. I've changed the apache port from 80 to 81 and when I go to localhost:81, I can see the main page of the ubuntu server; however, when I try to redirect to that address and port using the nginx reverse proxy (i.e. localhost/example_page) I get an 502 Bad Gateway error.

I'm trying to redirect to the apache server like this in my default file in the vhost.d folder:

location /example_page/{
  proxy_set_header Host $host;
  proxy_pass http://127.0.0.1:81/;
  proxy_connect_timeout 1200;
  proxy_send_timeout 1200;
  proxy_read_timeout 1200;
  send_timeout 1200;
}

Can you give me some ideas to make this work or tell me another alternative, please?

Thanks in advance.