Arduino Nano vs Micro: TX/ RX pin functionality

Hello! First post--I've read the posting guide but if I make a mistake it is due to ignorance not spite. Let me know!

TL;DR: Serial read code works on Micro (using Serial on Serial Monitor, Serial1 on Serial pins), but only partially works on Nano (using Serial on Serial Monitor, doesn't work using Serial on Serial pins).

I've been working on a project to make a serial display monitor. I have a device that outputs serial data at 57600 baud, and the project just reads that data (using Nick Gammon's excellent non-blocking code here), parses it, and outputs it to a Sparkfun LCD screen (like this).

I've been using an Arduino Micro for this project to debug, as I can easily send data over the serial monitor (using Serial) and make sure it appears on the screen, and then switch the code to use Serial1 for hardware testing. It all works great, with data appearing on the screen as intended.

However, I'd like to use a Nano going forward... but I cannot seem to read the Serial data through the hardware pins. My understanding is that on a Nano both the pinned Serial (RX/ TX pins) and the USB Serial are both "addressed" with a Serial command. With a Nano, I am able to pass my data through the Serial monitor and it appears as intended on the screen. Without changing anything (still "Serial" throughout code), using the TX/ RX hardware pins shows nothing. Attaching a flashing LED to the serial read doesn't blink the LED either.

As an example:

//#define SINGLE_SER

#ifdef SINGLE_SER
HardwareSerial & mySer = Serial; // for Nano, works with Serial Monitor not on Serial pins
#else
HardwareSerial & mySer = Serial1; // for Micro hardware
#endif

void setup ()
{
  mySer.begin(57600); //from batt/ monitor
} // end of setup

void loop()
{
  // if serial data available, process it
  while (mySer.available () > 0)
  {
    processIncomingByte (mySer.read ());
  }
} 

void processIncomingByte (const byte inByte)
{
~~blink led 13~~
}

Long story short, I'm not understanding something about the Arduino Nano's Serial pinning, but the wiring diagram doesn't show any switch that I can see... so what gives?

Any advice appreciated.

Did you swap Tx and Rx around by accident?