T-Call & PMU died after 10 days

[immutability]

I've seen several issue reports where a T-Call board couldn't be reprogrammed but mine seems to be a bit different, or maybe I'm doing something wrong. This is the first T-Call board I've programmed with a simple Arduino sketch to log temperatures every 2 minutes, using sleep in between readings, powered from a mains adapter (no battery, it was just a quick proof of concept).

The device worked nicely with stable readings and connectivity for 10 days - inside location in a cellar, stable climate with temperatures between 0 - 4 °C but then suddenly stopped. Connecting it to the serial monitor it outputs nothing, and no LEDs light up. Disconnected BMP280 from the I2C pins, removed SIM card, no change. Serial port is alive, and for 1 in 4-5 USB connections it would print the brownout detection to the COM port, but dying away - last line seemingly cut off, e.g.

19:08:42.172 -> Brownout detector was triggered
19:08:42.172 -> 
19:08:42.206 -> ets Jun  8 2016 00:22:57
19:08:42.206 -> 
19:08:42.206 -> rst:0xc (SW_CPU_RESET),b

It can't be reprogrammed from Arduino:

A fatal error occurred: Failed to connect to ESP32: Timed out waiting for packet header

I've tried multiple combinations of short and long-pressing both buttons (PWR and EN) on the board after and during power-on, with and without GPIO01 (pin marked "1" on the PCB) grounded, but no change in behavior, just the occasional brownout message. Serial monitor at 115200 baud. I've tried downloading the Espressif flash download tool but I'm not familiar with it and without the expected serial monitor lines after attempting to put the board into the download mode, I'm not sure if there's any point.

I guess I'm looking for guidance on what the most appropriate troubleshooting steps would be at this stage? Any voltages to measure? Different sequences of pin/button settings at startup to attempt to program it somehow? Thanks in advance!