Sounds like you might have dead spots in your house where your wifi isn't as good as it should be? I had the same and added a mesh router system (Netgear Orbi) and it helps a lot. Wifi extenders use different names for the extenders so you have to join each one, while the mesh system like the Orbi will use the same SSID so it's definitely seamless moving around the house and not losing signal.
Also, make sure to turn off wifi assist in your settings to avoid any unexpected data usage:
https://support.apple.com/en-us/HT205296