It’s more than just scaling. It will almost certainly require correct anchoring and potentially some grouping and layout elements.
Here are some notes on UI Anchoring, Scaling, CanvasScaler, etc:
Usually you need to choose a suitable ScaleMode and MatchMode in the Canvas Scaler and stick with it 100%. Generally if you change those settings you will often need to redo your UI entirely.
I also use this CanvasScalerOrientationDriver utility to make sharing UI for Landscape / Portrait easier. Read what it does carefully.