To use an on-screen keyboard as an input method it is required to install the Snapdragon Spaces Dual Render Fusion Package for Unity. The package can be downloaded it at Download | Snapdragon Spaces (qualcomm.com)  

At this moment, this package is not available for Unreal. 

After setting up Dual Render Fusion in a project, Unity Input Fields or Text Mesh Pro Input Fields that will trigger Android's on-screen keyboard as usual can be used on your phone. Documentation for Dual Render Fusion is available at Overview | Snapdragon Spaces™ Unity and Unreal SDKs (Qualcomm.com).