RE and DE are joined

One pair, one talker

Two wires carry one difference, so only one board can drive them at a time. If two drivers push A and B opposite ways at once, neither message survives. So every board spends almost all its time listening, and switches to talking only for as long as it takes to send.

The chip has a switch for each half:

• DE, driver enable. HIGH connects the driver to A and B.
• RE, receiver enable, active low. LOW connects the receiver to the TX pin.

Why they are joined

On this board a 0 Ω resistor, in the box printed RE-DE, joins the two. One pin now drives both, and because one is active high and the other active low, the pin always does exactly one thing:

That is what nearly every RS485 example sketch and library expects: one "direction" pin, called DE, DE/RE or TX_EN. Wire it to either hole; they are the same net.

While the board talks, its receiver is off, so its TX line would float. The board's 10 kΩ pull-up holds it high, which a UART reads as nothing arriving. So your sketch does not hear its own messages.

Set it first

Nothing on the board pulls the pin one way or the other. Between reset and the line in your sketch that sets it, the pin floats and the chip may switch its driver on. Set it LOW as the first thing setup() does:

const int DIR = 4;

void setup() {
  pinMode(DIR, OUTPUT);
  digitalWrite(DIR, LOW);   // listen until there is something to say
  // ...then start the UART
}

Taking the link off

Desolder the 0 Ω link and RE and DE become two separate holes. That is worth doing for one job: a self-test. Wire RE to GND and the receiver stays on while you send, so every byte comes straight back, which proves the chip's driver works with nothing else on the bus. For ordinary use, leave it fitted.