Just to go sure that I have understood your question right -> you have an internet connection but just on your tablet?
The easiest way it to create an hotspot - access it with your PC - download the engine over the launcher. After you have installed it the first time, you dont need any internet access anymore (just when you would like to update it) 
So you just have to download the launcher - then click on the engine install button and it will start another download for the engine.
When you dont want to do it with your launcher, you will have to use the source code engine : ?v=usjlNHPn-jo (you dont have to install git -> you can also just do it with the .zip file)