(TECH) RePhone with LED Matrix

Could you please send, how did you exactly connected the boards and the battery pack?

Just because the led matrix board must have survived your 3x AA batteries: it has a power regulator and it must work up to 6V! (I checked the circuit, datasheet of the regulator here farnell.com/datasheets/2012663.pdf)

Maybe you forgot to add a proper common ground, therefore the communication fails - hopefully.