Lines from all sides looks enough strange on iPhone X +.
Typically there is a top line only.
To make this line you can retrieve tabBar and to set shadowImage property. 
Looks simple, but you need to creare a image (not very comfortable).
IMO, more simple is to add a subview. For example:
	
	
	
	
	
	
	
	
	
		   Dim noTabControl As NativeObject = TabControl
    Dim noTabBar As NativeObject = noTabControl.GetField ("tabBar")
    Dim View As View = noTabBar    
    Dim Panel As Panel
    Panel.Initialize ("")
    Panel.Width = View.Width
    Panel.Height = 1
    Panel.Color = Colors.Red    
    noTabBar.RunMethod ("addSubview:", Array (Panel))