Indeed, calling Focus() method for RadEntry only focuses the input part of the control. In order to make the keyboard visible and select the text, you will need to implement it in separate custom renderers for each platform.
I have prepared a sample example with all changes that needs to be done. I created MyEntryRenderer.cs file for iOS and UWP platforms and EntryRenderer.cs file for Android platform with all required methods for making keyboard visible and selecting the text. In the Portable project inside MyEntry.cs file, MyEntry inherits from RadEntry.
For more details
on implementing custom renderers please refer to the Xamarin.Forms documentation https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/custom-renderer/entry
I am staying in touch for any further questions I could be of help with.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items