Android App connect to Mattermost team link

So I fixed my problem. don't know if this is what others are experiencing but in mattermost-ssl ssl_certificate points to cert.pem but should be changed to fullchain as in the documentation. there may be a difference in ciphers too. Here is what worked for me:

server {
   listen         80;
   server_name    mattermost.example.com;
   return         301 https://$server_name$request_uri;
}

server {
   listen 443 ssl;
   server_name mattermost.example.com;

   ssl on;
   ssl_certificate /etc/letsencrypt/live/yourdomainname/fullchain.pem;
   ssl_certificate_key /etc/letsencrypt/live/yourdomainname/privkey.pem;
   ssl_session_timeout 5m;
   ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
   ssl_ciphers 'EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH';
   ssl_prefer_server_ciphers on;
   ssl_session_cache shared:SSL:10m;

   location / {
      gzip off;
      proxy_set_header X-Forwarded-Ssl on;
      client_max_body_size 50M;
      proxy_set_header Upgrade $http_upgrade;
      proxy_set_header Connection "upgrade";
      proxy_set_header Host $http_host;
      proxy_set_header X-Real-IP $remote_addr;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header X-Forwarded-Proto $scheme;
      proxy_set_header X-Frame-Options SAMEORIGIN;
      proxy_pass http://app:APP_PORT;
   }
}

Could be other differences too like the SSL listen port. But using this with my domain name instead of just changing my domain name in the existing file works.