Found the solution, thanks to:
sudo certbot certonly --manual --preferred-challenges dns -d <yourdomain>
Then copy/paste the key given by Certbot into a record on the no-ip interface as described in that website.
The auto-renewal process seems like the next challenge, but one step at a time !
Thanks for your help and pointing out my errors & mis-understandings.
I'll switch this to resolved.
Cheers !