Hi @mystique28 ,
One way to increase internal storage is to configure the largest capacity SD card that you can afford as adoptable storage
You also need to update the phone to Android Version 6.0 (Marshmallow) to be able to do this.
You may already know of this way. If not, be sure to read the pros and cons of using your SD card this way.
Also not quite sure regarding the specifications of your phone as some sites say it supports 128GB micro SD card and this one states 256GB micro SD Card.
Whichever one is correct, it should support a 64GB SD Card, thereby giving you 80GB of 'internal' storage, if you use the adoptable storage method with this card.
Hopefully this is of some help.