Interfacing Aduino Nano and RDM8800
SoftwareSerial constructor is rx pin first then tx pin.

You have SoftwareSerial soft(2,3); But you said that

D3 - TX
D2 - RX


Which means soft rx is going to its rx and soft tx is going to its tx. These need to be swapped. Either swap the wires or change it to:

SoftwareSerial soft(3,2);