First of all, sorry for my poor English as I am writing from Switzerland.
As the ios app won’t let me see my files when I am out of my network, (I got the message for "non concording certificates), I bought a domain name to get a new certificate for it, but…
I try to get a Let’s Encrypt Certificate for my domain darkoflex.ch (and drive.darkoflex.ch).
As a result, I get an error message that tells : “Connexion to Let’s Encrypt failed. Make sure domain name is valid.”
When I am trying to connect to it now from outside, the browser let me know that my domain darkoflex.ch is using synology’s let’s encrypt certificate…
I already opened ports 80 and 403. DDNS set up done (I can access to my NAS using http).
The operating system is DSM (6.2.2 - 24922 - Update 4)
Isn't the second port for your redirect supposed to be port 443 and not 403?
Http opens your login page okay, but https times out.
Secure Connection Failed
An error occurred during a connection to darkoflex.ch. PR_END_OF_FILE_ERROR
The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
I might have found the problem: even though all ports are open in my NAS, I had to configure ports 80 and 443 in its firewall; after that, I could get my Let’s Encrypt certificate and now everything seems up and running!
A big thank to orangepizza, JuergenAuer and JimPas for help!