There are 3 things I would try:
Go to System Settings > Network, then choose the "..." dropdown menu and Locations. Make sure this is set to "Automatic"
The other thing I would try is to delete all known networks. This will require you to sign back into them with the password again. First shut off wifi in the menu bar, then go to System Settings > Network > WiFi > and click the Advanced button on the bottom of the page. In the Known Networks section, tap the circle with 3 dots on each network and "Remove from List". Turn the wifi back on and join your network.
The last thing is to go to System Settings > Network, then choose the "..." dropdown menu and "Set Service Order". Drag Wi-Fi to the top of the list.
Does your wifi connection drop at other times or only when put to sleep?