Letsencrypt (CertBot) with DynDns?

Hi all,
I try to use Letsencrypt: I forward 80 and 443 ports, I installed all the webservers (but I need Apache2 only) from dietpi-software but I always receive “No compatible and/or active webserver was found. Aborting.”. I use xxxx.dyndns.org (premium version), is that the problem? I don’t understand what I’m doing wrong.
Anyone can help me? Thanks in advance for all the replies!