EA scripts or Custom Indicators cannot be added to the MT4 or MT5 mobile phone app.
If you would like to run an EA (Expert Advisor) on your trading account, you will need to have a Windows, MAC , or iOS-based PC where you have installed the MT4 or MT5 terminal and have a 24-hour functioning Internet Connection for the EA script to be able to run uninterruptedly.
As long as you are running your EA on your PC device, you can follow the activities of your trading account on your mobile device.