Stuck on "Loading Data" from outside, fine on .local

I have the very latest HA installed on a RPi 3 with DuckDNS addon for outside access.

All was fine with external access until I had to change my wifi router. I have properly configured the new one with port forwarding, evidenced by the fact that I see the HA logo and “Loading Data”… but that is all I get!

When accessed internally via homeassistant.local or the direct IP address, everything is working correctly.

I’ve checked in the logs and can’t see anything obviously related to this, although I’m far from knowing what I’m looking for!

Any pointers gratefully received!

Clear your browser cache or use a private/incognito window.

Don’t know if you are using any reverse proxy, but if you do, also set this up accordingly.
HTTP - Home Assistant (home-assistant.io)

Has anyone figured out the answer to this issue? I am having the same exact problem. I used DuckDNS with Let’s Encrypt and nginx for what I understand to be secure. I can login locally with no issue. When I try to login externally it shows the home assistant logo and appears that it is loading. It even says “Loading Data.” But eventually it times out and says “unable to connect to Home Assistant. Retrying in XX seconds.”
Also, trying to use Incognito mode does not work. It blocks it completely.

Please help in anyway you can. I would like to get this set up and have already spent several hours of searching and trying new things.