Zephyr & PPP GSM Modem without RTS/CTS

Question

Hi, I have a problem configuring the PPP Modem correctly and I'm not quite sure what i'm missing here.

I'm running the code on a custom board. We are using nRF52832 to talk to Telit ML865G1 modem over UART and we don't have any HW control pins implemented so it's pretty much just RX&TX.

Everything seems to be functioning correctly but the response from modem is not being received by nRF52.

modem is powered and activated.
UART get correctly initialized
modem list (shell) lists the modem on the correct UART Iface
I scoped the UART lines and the AT command that's called by gsm_ppp_start->gsm_configure gets sent correctly with correct baud rate
The correct AT OK response is also generated by the modem and is detected both by a scope & a USB to serial converter
we also have an NRF5 sdk based solution that is able to successfully comunicate with the modem on the same board

however, with net_dbg logging enabled the modem generates the following error:

modem_gsm.gsm_configure: modem not ready -116

My suspicion is that the configurations i'm trying to run this in doesn't work unless RTS/CTS pins are implemented. Could you suggest something to troubleshoot this further or some other configuration combo that i'm missing here.

Code, board definitions and configuration provided below.

Here's a snippet of our custom board.dts:

 &uart0 {
	 status = "okay";
	 compatible = "nordic,nrf-uarte";
	 current-speed = <9600>;
	 tx-pin = <6>;
	 rx-pin = <8>;
	 label = "HTS_UART_0";
	 rts-pin = <0xFFFFFFFF>;
	 cts-pin = <0xFFFFFFFF>;
 };

I'm also using an additional overlay file with the following contents:

&uart0 {
	status = "okay";
	current-speed = <115200>;
	/delete-property/ rts-pin;
	/delete-property/ cts-pin;
	gsm: gsm-modem {
		compatible = "zephyr,gsm-ppp";
		label = "gsm_ppp";
		status = "okay";
	};
};

Here's the relevant bits of prj.conf:

CONFIG_UART_ASYNC_API=y

CONFIG_UART_0_ASYNC=y
CONFIG_UART_0_INTERRUPT_DRIVEN=n

CONFIG_MODEM=y
CONFIG_MODEM_GSM_PPP=y

CONFIG_NET_DRIVERS=y
CONFIG_NET_PPP=y
CONFIG_NET_L2_PPP=y
CONFIG_NET_NATIVE=y
CONFIG_NETWORKING=y

CONFIG_GSM_PPP_AUTOSTART=n

CONFIG_NET_PPP_ASYNC_UART=y

Here's the code itself:

const struct device *uart_dev = DEVICE_DT_GET(DT_BUS(DT_INST(0, zephyr_gsm_ppp)));
	gsm_dev = DEVICE_DT_GET(DT_INST(0, zephyr_gsm_ppp));

	LOG_INF("Board '%s' APN '%s' UART '%s' device %p (%s)",
		CONFIG_BOARD, CONFIG_MODEM_GSM_APN,
		DT_BUS_LABEL(DT_INST(0, zephyr_gsm_ppp)), uart_dev,
		gsm_dev->name);

	net_mgmt_init_event_callback(&mgmt_cb, event_handler,
				     NET_EVENT_L4_CONNECTED |
				     NET_EVENT_L4_DISCONNECTED);
	net_mgmt_add_event_callback(&mgmt_cb);

	LOG_INF("Modem Device INIT");

	gsm_ppp_start(gsm_dev);

It's pretty much a verbatim copy of net/gsm_modem example.