right config for letsencrypt

[bagagerek]

Hi y'all,

I want Nginx to run as a reverse proxy on my Rasberry pi with Motioneye.
I followed the manual but I can't seem tot get it right. I've forwarded 
port 8081 on my router.
My "sites-enabled" file looks like this:

server {
     listen 80;
     server_name mydomain.com;
     location /cams/ {
         proxy_pass http://192.168.178.244:8765/;
         proxy_read_timeout 120s;
         access_log off;
     }


}

When I put my local ip in the browser, I see the default nginxpage but 
when I put my external ipadress in the browser, I can't connect. Also  
if I go to mydomain.com I get an error saying the ip can't be found.
However, I can see my cam on mydomain.com:8081, but letsencrypt won't 
let me get an certificate, so I need this solved.
I've tried  to put "listen 8081" instead of "listen 80" in the 
sites-enabledfile, but when I reload nginx, I still can't connect.

Can anybody help me out here?

tx in advance