I am starting to have the exact same issues as you. My wireless SSID and network has not changed but I keep loosing connection to the controller and the only way to restore the connection is to power cycle the sprinkler controller. I am running Ubiquiti wireless access points in my house and I have no issues with all the other wireless devices in the house.
My biggest issue is when I loose connection to the controller the controller sprinkler schedule stops so my lawn ans flower garden dont get watered. My wife is about to kill me as I dont notice that it is not working until stuff starts to dry out. I bought the controller last year and had zero issues with it until this year a few weeks into watering season.
Hope there is a fix for this issue.