rtl_fm | aisdecoder is outdated, use rtl_ais instead.
The plugin manages rtl_ais running and stopping it as needed, you do not need a udp connection. New versions of the plugin do not need to invoke rtl_ais, as they link directly with it, rtl_ais is built into the plugin.
You can run rtl_ais, without the plugin manually, and then you do need to make a connection.
Most likely reason you cannot pick up ais
messages, is a bad antenna
and/or incorrect ppm correction.