[blush] Did that, and after 36 hours - no reboot. Will wait for a few days, just to be sure it won't restart after 56-60 hours, like Rahul_K mentions. Thanks.
Got exactly the same problem several months ago. Router keeps restarting (mostly on same 3 a.m.) on it's own. Model: RT-AC66U
https://pastebin.com/hYgpPmNU