AC68U regularly drops dhcp from ISP when inactive (sometime when active, too)

My RT-AC68U is currently sporting the stock ASUS 3.0.0.4.380_3264 firmware, but also previously used Merlin's 380.57 version. In both cases, the router regularly drops the dhcp address from my ISP (Shaw, in Canada) overnight, sometimes during the day (say after the lease expires, which is frequently only 1 hr), but the timing is inconsistent. Sometimes an hour, sometimes a few hours, sometimes when semi-active, and almost always when inactive for an extended period.

In either case I can't...

AC68U regularly drops dhcp from ISP when inactive (sometime when active, too)