We need more information.
Are you using the library that they provide?
What address are you using?
One thing that concerns me is that the library has support for s/w backlight
control but the board layout and the photo are showing a jumper to control
the backlight.
A clear closeup photo of the backpack you are actually using would help.
One that is good enough to be able to identify the i2c chip and the components on the
board.
There are multiple i2c chips out there and they work differently and need/use different libraries.
--- bill