This usually means the transmit power on your WiFi access points is too strong. The problem is that the way the current wifi protocol standards are written and implemented, most devices will just hang onto a wifi connection for as long as the connection is still functioning even if there is a superior alternate access point to connect to that's closer and faster. If you imagine drawing a map of your property, plotting the location of your wifi access points, and then drawing a virtual circle around them that represents wifi coverage, then you want as little overlap as possible between access points, and you do this by intelligent/strategic placement and by adjusting down the transmit power of the access points. There's free utilities you can download to your device to help you map out these rings. Although some less expensive and less configurable access points probably don't offer the the ability to change transmit power, prosumer and enterprise gear do. Some signal overlap between access points is unavoidable if you want to also completely eliminate all deadspots on your property, but with minimal overlap your device should be dropping a distant access point as you move out of range and pick up the closer one with the strong signal.

Most people think more power equals more better when it comes to access point signal strength but that's not really how it works because WiFi is 2-way communication and your mobile device is always gong to be the weak link because it has the weaker transmitter. There's no reason to broadcast a maximum strength signal from an access point if you have more than one of them.

Much has been written/documented on this topic and you should have enough keywords in the previous two paragraphs to find all the expert instructions for doing it that you could possibly need.