It's a DigitalOcean server.

After finish installation, I followed instructions on this URL
https://grandnode.com/how-to-install-free-ssl-certificate

cd /etc/nginx
mkdir ssl

cp /home/private.key /etc/nginx/ssl/private.key
cp /home/certificate.crt /etc/nginx/ssl/certificate.crt

Now, I am trying to modify default
vi /etc/nginx/sites-available/default

I added the following in server section after         listen 80 default_server;         listen [::]:80 default_server;

       listen 443 ssl default_server;
        listen [::]:443 ssl default_server;

        ssl_certificate /etc/nginx/ssl/certificate.crt;
        ssl_certificate_key /etc/nginx/ssl/private.key;


I saved it.

nginx -t
systemctl reload nginx
vi /var/webapps/grandnode/App_Data/appsettings.json

I made only one change here
"UseForwardedHeaders": true

Then I rebooted Ununtu server. Restarted Grandnode.
My browser gives me ERR_CONNECTION_REFUSED when I tried to connect https://mystore.com

I'd appreciate it if someone can give me a hint.

Thanks.

[/b][b]

I have turned on https on port 443 by using command
sudo ufw allow http
sudo ufw allow https

ufw status verbose

22/tcp                     ALLOW IN    Anywhere                  
80/tcp                     ALLOW IN    Anywhere                  
443/tcp                    ALLOW IN    Anywhere                  
443                        ALLOW IN    Anywhere                  
22/tcp (v6)                ALLOW IN    Anywhere (v6)            
80/tcp (v6)                ALLOW IN    Anywhere (v6)            
443/tcp (v6)               ALLOW IN    Anywhere (v6)            
443 (v6)                   ALLOW IN    Anywhere (v6)    

Finally I solved the problem by myself

vi /etc/nginx/conf.d/default.conf

added the following

   listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/mysite.mydomain.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/mysite.mydomain.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

Wow! It's working.