If there's no global connection available for port 80 then Let's Encrypt can't connect to authenticate via http-01.
Personally, I highly recommend that you use dns-01 authentication via the certbot duckdns plugin for which I gave you a link before. It will avoid any IP address complications completely.
Hi Griffin,
I know, if there's no 80(443) connection to the outside there is no way.
Finally I was successful in solving the problem. My idea in the beginning that there's something wrong to the ip-forwarding in my router was the right trace.
So finally I choosed an other online station to receive the traffic and reroute it to the server's secondary ip and -god thanks- that workaround is doing the job.
So my dear, thank You very much for Your help and patience and as we say in German: it was helpful, that we talked about it