Help with SSL certs, please

I’m a touch confused. I had a power outtage issue on Monday, lots of on and offs, and then when I got power back, HA was fine but I’m having what I think are SSL issues but the timing might be a coincidence. A touch of history - my HA used to run with the DuckDNS plugin. I have setup Alexa voice control the complex way with an AWS account, Lambda etc. This all worked fine and every 6 months or so I had to disable the Alexa skill (via my phone app) and re-enable it to keep voice working. All good - documented by me and always worked.

A few months ago, due to poor voice control performance, I pinned it down to poor DuckDNS performance, and installed Dynu DNS addon . All worked Perfectly! (dynu is nice and fast.)

Now - alexa integration has died, and my Alexa app on my phone point blank won’t re-enable. No matter - I’ll try the Hue emulator - but that’s for another day.

I also noticed my phone app won’t connect, with the “certifical mismatch error”. So I investigated a bit.

On my PC, the website for HA (local) 192.168…yada… shows as insecure - and thats never caused a problem.

Today I clicked on the warning “not secure” and it shows “cert details” as being “xxxxxx.duckdns.org” - although I’m now using Dynu DNS…

I’ve gone into HA system/network settings, and I “did” have the “local network error” - but I’ve now changed that to “https://homeassistant.local”. Makes no difference.

My home assistant URL in these settings is. “https://xxxxxxx.freeddns.org:8123

I just don’t understand what’s changed, and where I need to “install” a new “correct SSL cert” - I’m assuming this is why Amazon can’t connect. And why my app wont’ connect.

Is there a plugin I need to install? I’ll be honest, the whole SSL cert thing confuses me and I’ve no idea how and where I install one.