I looked all over for an appropriate '.conf' file, but to no avail. As a last resort, one can always create a service script that turns power management off at boot. Try the following: make a file named something like 'wifi-power-management-off.service' in '/lib/systemd/system' that has the following content:
[Unit]
Decription=Disable power management for wlan0
Requires=sys-subsystem-net-devices-wlan0.device
After=sys-subsystem-net-devices-wlan0.device
[Service]
Type=oneshot
ExecStart=/sbin/iwconfig wlan0 power off
Then issue the command 'systemctl start wifi-power-management-off.service' and check to see if it worked by doing a 'iwconfig'. If it works, commit the service to execute at boot with 'systemctl enable wifi-power-management-off'. I think that should work.
Jim