I use an Apple Airport Extreme and two Apple Airport Express devices for my WiFi Network. The two Apple Express devices are set up as range extenders. This setup works well, with the exception of when I use my Microwave Oven the newer and furthest Airport Express drops the connection to any devices that were using it. I have to unplug that Express and then the devices reconnect. I am sharing this only to say that interference can play a part in Network issues with Range Extenders. My newer Airport Express is a Dual Mode supporting 2.4 and 5.0 at the same time. Any connected device capable of 5.0 will use the faster signal.
Don't know if your Network devices allow for manual Channel selection but if so you might want to try some various channels to see if that offers a reliable connection.