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Cellular firmware is not hard to write. It is hard to bring up. The modem sits there not attaching, and the reason lives inside the modem, not in your log. Adsum ships two tools for exactly that, and knowledge for the three development kits, curated by Omar Morceli.
Needs a free account, registered once with GitHub or email, as LoRa and satellite do; BLE, Wi-Fi and Ethernet don’t. Signed, licensed LEW840x cellular firmware is on gateway firmware and the cellular LEW840x page.
Adsum IoT Coder home with an nRF9151 DK detected, prompting registration to unlock cellular attach recipes

An nRF9151 DK detected, no account yet: the home names what registering unlocks.

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What is supported

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LTE-M or NB-IoT

A configuration decision made before writing code. Wrong, it costs a week.
Neither works if your operator does not run it in that cell. Check coverage before blaming the firmware. It is the most common cause of a modem that will not attach.

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Two tools for bring-up

The board shell speaks AT commands to the modem and Zephyr commands to the application, over the serial link you already have. When that is not enough, the modem trace reads the signalling inside the modem and names the step that failed: A bug in your own code is what 3-layer debug is for on the radio side.

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GNSS and power

GNSS shares one RF path with LTE, so an aggressive report interval starves the fix. Assistance data turns minutes into seconds. Battery life is set by how the modem sleeps: PSM and eDRX are requested, the network decides what is granted, and the shell shows the answer.
Satellite NB-NTN knowledge ships too, and runs only on an nRF9151 of the LACA A1A revision, not A0A, the nRF9161, nRF9131 or nRF9160. Check the module label first. See satellite IoT, and the Adsum NTN Relay, which carries it.

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Starting a cellular project

1

Name the board and the behaviour

The board knowledge loads with it, then settle LTE-M or NB-IoT before writing code.
2

Build, flash, attach

Confirm with the board shell rather than guessing from logs; a stuck attach gets a trace.
3

Treat it as a product

A connected device is in scope for the Cyber Resilience Act.