How to access NextCloud from outside and from inside my LAN

Thank you @tflidd !
Yes, it would be great if I could do everything with Gandi.net. without using DuckDNS.

So far I have managed to create a CNAME record so that I can access NextCloud with http://cloud.mydomain.com. And it’s not a web redirection.
So, we’re progressing !
Now, there are still a few questions:

  1. I don’t know how to change the settings on my Android NextCloud and DAVx5 apps (because now I don’t need to use mycloud.duckns.org anymore, do I?).
    Do I have to uninstall the apps and install them back again?
  2. Now Gandi.net sent me the link to this script:
    https://github.com/rmarchant/gandi-ddns
    and its documentation:
    https://api.gandi.net/docs/livedns/ .
    So I am studying this. It should allow me to by-pass Duckdns.