Asus RT-AC86U shutting down at night?

I recently installed an RT-AC86U (following recommendations in another thread) and it has been working great. Except... a few times in the last couple of weeks my internet has gone down in the middle of the night. I know this because my burglar alarm (which is reliant on a wifi connection) has emailed me to say that it was offline for a few hours in the night.

How can I check if my router is the...

Asus RT-AC86U shutting down at night?