SSL Behind DDNS (xxx.dyndns.org)

I’ve gotten conflicting answers on if I can apply a LetsEncrypt SSL on a DDNS domain like I am using on my local network (xxx.dyndns.org).
Can this be done?
If not, how do I use the self-signed SSL to get rid of the warnings in my browser?

I’ve been experimenting with NextCloud on an old laptop with Linux Mint 19.2.
I installed NextCloud server using snap (sudo snap install nextcloud). That was easy and went very well.
I set up port forwarding on my router, and all seems good. I can access the NextCloud server outside of my LAN.

Then, I tried to setup SSL with LetsEncrypt following these directions (Option 1), but that failed.
After that failed, I set up a self signed SSL following the Option 2 directions.
The self-signed method worked, but I get warnings in my browser. How can I eliminate those browswer warnings?

did you forward port 80 and 443? port 80 is needed for letsencrypt and blocked by some isps.

https://www.google.com/search?q=self+signed+certificate+chrome

I’m not sure specifically about dyndns, but people have had success using letsencrypt with other dynamic DNS services such as duckdns.

Yes, port forwarding is setup. See screenshot of my settings below.

As for eliminating the browser warnings, I followed these steps, rebooted my computer, but I still get the ‘not secure’ warnings and I do not get the padlock in the address bar.

and you are able to access nextcloud via http?
what was the letsencrypt error message?

Yes. After install & port forward was setup, I could access nextcloud via http.
Then, I tried letsencrypt which failed. So, I applied the self-signed SSL.
Now, I can access nextcould via https, but I get the aforementioned warnings.

Sorry, I don’t remember. And I did not save the message output.
I might be able retry and make note of the error.

GOOD NEWS!!! Letsencrypt SSL works for me!
I intended to reproduce the error I got the first time I tried to set up letsencrypt SSL tonight.
So, I went through the steps… but did not get an error!
I opened my browser and VOILA! I don’t see any warnings, and there is a locked padlock in my address bar! WOO! HOO! YAY!
Sorry for the noise.

Here is a copy of my console with exactly what I did and the results (names changed for security):