http /How to turn external SD card into internal storage on 'Marshmallow'. You would need to post in the 'Rooting, Jailbreaking & Unlocking' Forum.
If you have rooted the phone, that I can't help with. Now if you can upgrade to 'Marshmallow', the instructions for setting up the SD card correctly are included below. Your device should only run up to 'Lollipop'. You would need to be running at least 'Marshmallow' to get that to work. Unless that phone has been rooted, it shouldn't allow for running apps off the SD card.
