I use my Synology almost every day and for the past month I wasn’t able to connect in to it externally though HTTPS port 80 or 443. I tried a few things on my router but I was still getting a Connection Refused error in Chrome and Internet Explorer would say it cant load the page.
I ended up ruling out my router by moving the NAT rule for ports 80 and 443 to another server on my LAN, which worked, so I knew it wasn’t the router or rules that I change often. I found a post saying to disable the QuickConnect, restart the Synology, then enable it again.
It took two tries and sometimes redirecting to the port 5001 doesn’t work but now CloudStation is able to connect offsite.
I also found this post saying to make sure that HTTPs service is enabled.