Can't get Komga OPDS working

I’ve got a Komga install with OPDS running successfully. I know iOS currently only supports OPDS via HTTPS, so I setup a reverse proxy for https://komga.[mydomain].com and confirmed it’s working. When I go into Panels to setup OPDS, I can’t get it to work. I’ve tried using port 443 (what I think should work), as well as 80 and blank. The error I always get is:

Unable to find Server
We were not able to find your OPDS Server. The following error was found: The request timed out.

Any ideas?

Can you access your server via web browser?