nRF9151 uart_callback_set returning ENOSYS

Question

Hi there, I am trying to implement a UART handler on my nRF9151 that can Tx GNSS updates to my nRF52833. I am adapting this into the cellular/gnss sample, which is working completely fine on its own. However, when I try to run uart_callback_set(), I am getting -88 in return.

I have followed the Async UART on Nordic Academy to the best of my abilities.

Here is my UART handler. Right now, I am just trying to set up something basic to echo back when it receives an ECHO message from my nRF52833.

// nRF52833 UART Handler

#define NRF_UART_BUF_SIZE 128
#define RECEIVE_TIMEOUT 100
static char uart_rx_buffer[NRF_UART_BUF_SIZE];
static size_t uart_rx_index = 0;
static uint8_t rx_buf[NRF_UART_BUF_SIZE] = {0};

static void nrf52833uart_cb(const struct device *dev, struct uart_event *evt, void *user_data)
{
    switch (evt->type) {
    case UART_RX_RDY: {
        const uint8_t *data = &evt->data.rx.buf[evt->data.rx.offset];
        size_t len = evt->data.rx.len;

        for (size_t i = 0; i < len; i++) {
            if (uart_rx_index < NRF_UART_BUF_SIZE - 1) {
                uart_rx_buffer[uart_rx_index++] = data[i];
            }

            // Check for newline or carriage return to end message
            if (data[i] == '\n' || data[i] == '\r') {
                uart_rx_buffer[uart_rx_index] = '\0';  // Null terminate string

                // Print the full received message
                LOG_INF("UART RX message: %s", uart_rx_buffer);

				if (strcmp((char *)rx_buf, "ECHO") == 0) {
					uart_send_async("nRF9151 ECHO!\n");
				}

                // Reset index for next message
                uart_rx_index = 0;
            }

        }
        break;
    }
    case UART_RX_DISABLED:
        // Restart RX (adjust rx_buf and timeout as per your code)
        uart_rx_enable(dev, rx_buf, sizeof(rx_buf), RECEIVE_TIMEOUT);
        break;
	case UART_TX_DONE:
		uart_tx_busy = false;
		break;
    default:
        break;
    }
}

This is how it is called in main.

int main(void)
{	
	int err;
	uint8_t cnt = 0;
	struct nrf_modem_gnss_nmea_data_frame *nmea_data;

	LOG_INF("Checking nRF52833 UART connection");
	k_sleep(K_MSEC(10));
	if (device_is_ready(nrf52833uart_dev)) {
		LOG_INF("nRF9151 UART connection is ready. Setting up UART callback protocol.");

		int ret = 0;
		ret = uart_callback_set(nrf52833uart_dev, nrf52833uart_cb, NULL);
		if (ret) {
			LOG_ERR("Failed to set up UART callback protocol: %d", ret);
		} else {
			ret = uart_rx_enable(nrf52833uart_dev ,rx_buf,sizeof rx_buf,RECEIVE_TIMEOUT);
			if (ret) {
				LOG_ERR("Failed to enable UART RX: %d", ret);
			}
		}
	} else {
		LOG_ERR("nRF9151 UART connection is not ready");
	}

I have set:

CONFIG_UART_ASYNC_API=y

CONFIG_NRFX_UARTE0=y.

#define NRF52833UART_NODE DT_NODELABEL(uart0)

const struct device *nrf52833uart_dev = DEVICE_DT_GET(NRF52833UART_NODE);

I've also tried CONFIG_TFM_LOG_LEVEL_SILENCE=y to try and release uart0.

Thank you!