Should work!

manifests/init.pp

@@ -5,9 +5,7 @@ class ai4ca (
   include profiles::letsencrypt
   include nginx
 
-  $server_name = fact('fqdn')
-
-  nginx::resource::server { $server_name:
+  nginx::resource::server { fact('networking.fqdn'):
     ensure              => present,
     www_root            => $www_root,
     location_cfg_append => {
@@ -15,14 +13,14 @@ class ai4ca (
     },
   }
 
-  nginx::resource::server { "${server_name} HTTPS":
+  nginx::resource::server { "${fact('networking.fqdn')} HTTPS":
     ensure      => present,
     listen_port => 443,
     www_root    => $www_root,
     index_files => $index_files,
     ssl         => true,
-    ssl_cert    => fact("letsencrypt_certs.${server_name}.files.combined"),
-    ssl_key     => fact("letsencrypt_certs.${server_name}.files.key"),
+    ssl_cert    => fact("letsencrypt_certs.${fact('networking.fqdn')}.files.combined"),
+    ssl_key     => fact("letsencrypt_certs.${fact('networking.fqdn')}.files.key"),
   }
 
   firewalld_service {