Help to configure Foreman/Puppet to use hostname instead of FQDN

I need help to configure Foreman/Puppet to use a hostname instead of FQDN. It's a base installation on one single server via foreman-installer. No matter what I write into the puppet.conf file on the client, there is always the FQDN being used in Foreman. Problem: If I change the network, then Foreman shows up that a new client has been found and asks to sign it. But in fact it is the same machine. Best solution would be to "hardcode" a client name in puppet.conf. But for some reason neither "certname" nor "node_name" works...