Add mesinput app from the store, then go in and select the VNC empty keyboard and then access it by swiping among the installed keyboards (on the keyboard itself) to get to the empty one.

This will display nearly the entire screen for use with a bluetooth. My success with the btinput app is hit & miss. At times it flawlessly works and then there are times that I have to reconnect/disconnect numerous times.

When it works, it works beautifully and I have two keyboards that I use. One is a folding type and the other is the MobileKeyboard6000 from Microsoft.