When using the Raspberry Pi over marginal Wi-Fi links I have experienced problems with the Pi dropping the network entirely. When that happens the only fix is a reboot. After much searching, it would appear that the Wi-Fi power save feature may be contributing to the problem. The Wi-Fi power save is enabled by default but can be temporarily disabled by entering:

iw wlan0 set power_save off

To make the change permanent, open a terminal session and use the following commands to edit the rc.local file.

sudo nano /etc/rc.local

In a blank line above exit 0, enter the following:

/sbin/iw dev wlan0 set power_save off

Next press Ctl+X followed by Enter to exit and save the file
That completes the changes and you can reboot the Pi. To check that the wlan power save is disabled use the following command:

iw wlan0 get power_save