I think you've an issue with the way you've define your UI scaling.

The best way is to set your canvas to scale with screen size and define a reference resolution according to the one in your editor/simulator. I also recommend to set the scale on match height only for a landscape game and width for portrait.

You can then set anchors to define where each element will try to place itself.

Find more information there : https://docs.unity3d.com/Packages/[email protected]/manual/HOWTO-UIMultiResolution.html