Post mortem analysis ?

[Eric73]

Hello, after three month of 24/24 working my lopy4 (firmware 1.18) have stopped to send sigfox message. As i am far from board location (300km) i cannot know if it's a hardware failure or a software failure.
Lopy4 is fitted on a board i have designed powered on main supply and mainly do some bluetooth communication every 1min with several bluetooth sensor and every 15min send data collected by sigfox.

Between two bluetooth scan period i do a machine.idle() call. I never call gc.collect() and i haven't started watchdog (may be on the next software update i will add it).

So my questions are :
-When i go to the board location, is there any way to know what happened prevously ? eg is there any way to have the last exeception trace that previously occurs ?
-What is the maximum time i can use for WDT instance can i do a

wdt = WDT(timeout=120000)

or even better a

wdt = WDT(timeout=900000)

that will permit me to refresh the watchdog after a sigfox send ?
-How can i have via REPL the last reset cause of the board to see if a watchdog reset or power failure occurs between last REPL connection time?

Thanks for your ideas and feedbacks