LIN Network without Transceiver (for testing)
How do you know that you have any communication working? If communication does not work with direct TX->RX connections, then it will also not work with multiple nodes and LIN transceivers.

You need to get communication running between 2 nodes and add more later.

Connect a USB to UART cable to the RX&amp;TX lines and check the data. If that does not work - connect a logic analyzer to see that your timing/baud makes sense. If that does not work - configure the pin as GPIO and see if it works at all.