Without a data line you can’t, solutions were provided.
You need to look into a ESP or other type PCB that allows you to read the voltages of the batteries to then send that data into your home assistant instance. Someone with more experience will be best to answer in this regard.
The NAS is tied via the integration and if there is no integration use a smart plug tied to the timer of the main plug being used for the detection of power loss.
edit: a quick look up for Terramaster NAS’s gives this post