I have disabled the following, because I don’t want to use any third party.
Both client are up to date Windows Client (SyncTrazor). On my side, I have made a port forward for port 22000 and I have a DDNS-Address. The other side was using a VPN-Provider and was not reachable from the internet.
In that client we changed the address of mine to my DDNS-Address and also the settings shown above. It wouldn’t work. But if we changed it to my IP-Address, it does work. That system had no problem with DNS in general and could find my corresponding ip-address.
So is this expected or is it a bug?