In Synology when you create a website in web portal, lets say mywebsite.eu you can access the site with http or https depending on your settings like so:
But if you want http traffic for the site to go directly to https you can do the following
Go to Web Station -> Web Service Portal -> click on the host mywebsite.eu and go to Edit -> Find HTTPS Settings and Enable HSTS
Now wen you open http://mywebsite.eu it will redirect you directly to https://mywebsite.eu