D6220 keeping wifi device awake burning battery

I have a D6220 router. I've had it a while. Only recently did I discover that the cause of a problem that I've been having is caused by the router's handling of the wifi network. (How long it's been happening and how long I've had the router are comparable.)

The problem is that my phone, when connected to the wifi, burns through the battery at record speed. In any other circumstances, it lasts almost 48 hours, but attached to the D6220, I'm lucky to get 10 hours before it is toast.

The information I found is that this has something to do with a keep-connected signal sent by the router.

The question is: where is the setting to turn this battery drain off?

(I have just updated firmware to 1.0.0.66)