UART works on a custom board with nrf52840, but not on the nrf52840 DK


I have a custom board with nrf52840 chipset that controls a GSM modem over UART, and this setup works just fine (zephyr project). 

Now, I'm trying to replicate this setup with an nrf52840DK board controlling dev kit of the same modem, but I don't see any activity on the UART Tx line. 

I'm using the exact same project that works fine on the custom board. Are there any specific configurations that need to be set when using the DK?