The schematic shown here: RAK3172 Evaluation Board Datasheet | RAKwireless Documentation Center
Pins 4 and 5 of the RAK3172 are labelled UART2_TX and UART2_RX - these should be UART1_TX and UART1_RX.
Pins 1 and 2 are already shown as UART2_TX and UART2_RX.