Serial LTE Modem AT commands over HW Serial

Question

Hi I'm trying to load a custom board with nrf9160 with the Serial LTE Modem, and route the AT commands to the UART from a coprocessor (ESP32C3) I have followed the guide on using an external MCU: https://docs.nordicsemi.com/bundle/ncs-2.9.0/page/nrf/applications/serial_lte_modem/doc/slm_description.html But its not clear as to whether I need the 52840 chip on my custom board for this to work. The closest I have got to this working after several nights of modifying overlays is access to the shell over my HW Serial port, but none of the AT commands are available. Here's a rough breakdown of the steps I've taken: 
 Background: I’m using the Nordic nRF9160 DK with the Serial LTE Modem (SLM) sample from NCS v3.0.1. My goal is to route the AT-command interface (UART1?) to Arduino header pins P0.05 (RX) and P0.06 (TX), then get the SLM shell working over that serial port. 
 1. Pin-mux & Overlay

By default, UART1 was mapped to P0.14/P0.15. I created a custom DTS overlay ( nrf9160dk_nrf9160_ns.overlay ) to remap uart1_default to pins 5/6.

Confirmed via the graphical devicetree viewer that the pins changed and, after rebuilding, saw “UART test: Hello from P0.05/P0.06!” on the serial console.

2. Shell vs. Console Confusion

I tried driving AT commands and shell commands directly over UART1, but:

In “console” mode I got AT: command not found .

In “shell” mode I got a Zephyr shell prompt but none of the SLM AT commands (e.g. AT+CFUN? ) were recognized.

Turns out SLM uses its own AT parser/backend, not the stock Zephyr shell, so I needed to disable the Zephyr shell and enable the AT console backend.

3. prj.conf Tweaks & Kconfig Errors 
 I modified prj.conf heavily, enabling/disabling for:

CONFIG_SHELL , CONFIG_SHELL_BACKEND_SERIAL

CONFIG_UART_CONSOLE vs. CONFIG_RTT_CONSOLE

CONFIG_AT_CMD_CUSTOM , CONFIG_NRFX_TIMER2 , etc.

Enabled downloader, FOTA, MQTT, TF-M lazy stacking, etc.

But that introduced a flood of build errors:

Undefined symbols: AT_CMD , AT_HOST , TFM_ENABLE_FP , TFM_LAZY_STACKING

Missing nrf_cloud_agnss_schema_v1.h , cJSON.h

Missing downloader sizes: CONFIG_DOWNLOADER_MAX_HOSTNAME_SIZE , etc.

MQTT config undefined ( CONFIG_MQTT_CLEAN_SESSION )

Dependency loops in TF-M config: “INVALID CONFIG… AND CONFIG_TFM_LAZY_STACKING”

Errors in slm_at_nrfcloud.c , slm_at_fota.c , slm_at_gnss.c , main.c , etc., due to missing headers and undefined Kconfig symbols.

4. Root Cause & Recovery Plan 
 I’d (re)discover that:

SLM has its own AT backends —you don’t use CONFIG_SHELL but rather the AT console ( CONFIG_AT_CMD_CUSTOM , CONFIG_LOG_BACKEND_UART , etc.).

Only pull in the exact downloader/MQTT/Kconfig symbols you need—don’t blanket-enable unsupported features.

Revert to a known-good commit and then incrementally re-apply each config line, rebuilding after each to catch exactly which setting breaks the build.

Next Steps:

Forum Request: Anyone who’s successfully routed UART1 → P0.05/P0.06 for SLM, could you share your minimal prj.conf and overlay snippets?

DT Overlay Example: What exact lines did you use in boards/nrf9160dk_nrf9160_ns.overlay to get Zephyr’s uart1 working on Arduino pins 5/6?

Kconfig Tips: Best practices for including downloader/FOTA/MQTT symbols in SLM without pulling in every network feature and blowing up TF-M?

Many thanks in advance for reading this query and any insightful responses! Tom