Hello, 

kohcocrunch

I faced the same issue on-site as well.

The tx rx led is not lid.

We power cycle the whole modbus module then the the tx rx light starts to blink.

The comm then recovered.

looks like your code missing some diagnostic steps to know at what exactly part of communication process is actually. Your code looks like hangs at some procedure with specific ERROR with STATUS or other value what you need correctly handle in your code to be sure about steps your code has. Therefore only initialisation is your last help for functionality again.