Can't open GUI with FQDN but can with IP address

I set up DuckDNS and Let’sEncrypt about a year ago following the recommended method. I did NOT use nginx (reverse proxy). It worked fine for months, then I didn’t use it for a while. Recently I updated to 2021.10.6 (from maybe 2021.06?) and updated all addons. I then tried to access the GUI from a computer on the same subnet using the FQDN and it gives “Unable to connect to Home Assistant. Retry”. From the IOS app I get NWError error 2 and when editing the internal URL I get “The certificate for this server is invalid…”

So from the computer I tried using the IP address and it worked. Then I used nslookup to verify the DNS. nslookup gave exactly the same address though the address works and the (correct) FQDN doesn’t.

Now I see that in July (I think) there was a breaking change for those who use rev proxy. You have to add entries in the http: section of configuration.yaml. I don’t use rev proxy so it shouldn’t apply but I’m not sure.

So I want to know a few things:

  1. why is there an Internal_URL and an External_URL available in HTTP? What does this do? Why does http have to know?
  2. what certificate is the app complaining of?
  3. how do I fix this.

Mostly, I thrive on understanding so I can fix things myself when they fail as they will again. I would love to know, not just (hypothetical example) “enter this as the Internal_URL” but rather “when the HTTP server receives a request it checks the header field to see if it matches the Internal_URL”. So I’m saying I would love any help but especially that which comes with how it works, not just a howto.

TIA
Keith

I can see one interesting thing. The ssl/fullchain.pem hasn’t updated since June 26 and is now expired. Don’t have a clue why. Maybe I did the breaking update to HA after June 26? Don’t really know what the cert is for, what checks it and, still, why I can get in with the ip address. Does IP address bypass certs?

Basically, I have no idea what the validation chain is. If I did, I probably could fix this.