Hi Matthew,
Just to confirm, you are using a custom board, is this correct? If so, could you please confirm that you followed these guides for your hardware design?
JN5189-Power Consumption
JN5189 modules development reference manual
Could you please help us providing more details about the problem you are facing?
As you mentioned, you can take a look at the powerdown_demo project, inside the J5189 SDK, and use it as a base to develop your application. More information on power management can be found in the JN5189 User Manual (UM11138), Chapter 5: Power Management.
Also, we recommend to update your SDK version. A newer version is available in the MCUXpresso SDK Builder.
Regards,
Eduardo.