Let's encrypt cannot validate this domain name. Please make sure port 80 of your Synology NAS and router is open for LE domain validation over the Internet
All other communication with LE will be over https to protect your Synology NAS.
that's just complaining about port 80 not open to internet:
I don't think the ISP Freedom Internet itself has geoblocking.. (Can't find it in their management panel.) So it must be some on-premise device doing the blocking.