Seeing people recommend nginx proxy manager, I’ve tried to set this up but never managed to get the certificates to work from letsencrypt (“internal server error” when trying to get one). When I finally got it working a while ago (I think I imported a cert), any proxy I tried to setup just sent me to the Synology login page.
I think WebStation is causing this. I just investigated my Synology NAS and discovered that the default web portal is redirecting ports 80 and 443 to the synology login portal (which lives in ports 5000 and 5001 depending on whether you use SSL or not.)