Why my XBee S2C STM transmits packets with USB but not with UART

So I have been working with an XBee S2C SMT module in API mode with zigbee sensors connected and packets being transferred between. However, when I connect my module to the UART of my development board, I don’t see the packets anymore. Here’s my pinouts:
XBee > Dev Board
VCC > 3.3V
GND > GND
DOUT > RX
DIN > TX

Following that pinout and then accessing the UART port doesn’t seem to work. If I connect to using a USB-to-TTL instead, everything seems to work fine and I am able to see the packets with this setup:
XBee > USB-to-TTL > USB0 on Dev Board
VCC > 3.3V
GND > GND
DOUT > RX
DIN > TX

I don’t understand why the USB setup works and the UART setup doesn’t. The XBee module I am using is XB24CZ7PIS-004. Any help would be greatly appreciated!