I have done this on Challenger LTE board [link](https://ilabs.se/product/challenger-rp2040-lte/), which is RP2040 based.

You need to enable data serial at boot (`boot.py`), this is disabled by default. When the board boots you need to run file `serial_passthrough.py` 
the program.

Your computer will report new COM/Serial port. Use that to talk with the device connected to `uart` of the CircuitPython board.

## Create files on CIRCUITPY
file: `boot.py`
```python
import usb_cdc

usb_cdc.enable(console=True, data=True)
```
Next creat `serial_passthrough.py` file.

file: `serial_passthrough.py`
```python
import board
import usb_cdc

uart = board.UART()
usb = usb_cdc.data

while True:
    if uart.in_waiting:
        data = uart.read(uart.in_waiting)
        usb.write(data)
    if usb.in_waiting:
        data = usb.read(usb.in_waiting)
        uart.write(data)
        
 ```
 
 If you want to talk with Ublox Sara-R410M module as I would like to. 
 
 `serial_passthrough_sara.py` would be like this:
 
 file: `serial_passthrough_sara.py`
 
 ```python
 import board
import busio
import digitalio
import usb_cdc
import time

uart = busio.UART(tx=board.SARA_TX, rx=board.SARA_RX, rts=board.SARA_RTS, cts=board.SARA_CTS, baudrate=115200, timeout=0.25)
usb = usb_cdc.data


# SARA LDO enable control signal
sara_pwr = digitalio.DigitalInOut(board.SARA_PWR)
sara_pwr.direction = digitalio.Direction.OUTPUT
# Make sure the modem is fully restarted
sara_pwr.value = 0
time.sleep(1)
sara_pwr.value = 1

# Power on button
sara_btn = digitalio.DigitalInOut(board.SARA_BTN)
sara_btn.direction = digitalio.Direction.INPUT
sara_btn.pull = digitalio.Pull.UP

# Reset pin
sara_rst = digitalio.DigitalInOut(board.SARA_RST)
sara_rst.direction = digitalio.Direction.INPUT
sara_rst.pull = digitalio.Pull.UP

# Perform a SARA power on sequence
print ("Powering the SARA modem on.")
sara_btn.direction = digitalio.Direction.OUTPUT
sara_btn.value = 0
time.sleep(0.15)
sara_btn.direction = digitalio.Direction.INPUT
sara_btn.pull = digitalio.Pull.UP
# A short delay is required here to allow the modem to startup
time.sleep(1)
print ("Reset done, waiting for modem to start.")


while True:
    if uart.in_waiting:
        data = uart.read(uart.in_waiting)
        usb.write(data)
    if usb.in_waiting:
        data = usb.read(usb.in_waiting)
        uart.write(data)

```